Corrosion Prevention

By retarding either the anodic or cathodic reactions the rate of corrosion can be reduced. This can be achieved in several ways :
4.3.1 Conditioning the Metal
This can be sub-divided into two main groups:
(a) Coating the metal, in order to interpose a corrosion resistant coating between metal and environment. The coating may consist of:
(i) another metal , e.g. zinc or tin coatings on steel,
(ii) a protective coating derived from the metal itself,
e.g. aluminium oxide on “anodised” aluminium,
(iii) organic coatings, such as resins, plastics, paints, enamel, oils and greases.
The action of protective coatings is often more complex than simply providing a barrier between metal and environment. Paints may contain a corrosion inhibitor
ALSO……
Underbody structural components are typically coated to provide a first line of defense against corrosion. For light truck frames, the two most common coatings are hot melt wax and electrocoat (E-coat). Paints are also used on current light truck frames. Conversion coatings enhance the adhesion of electrocoat or paint, and they are commonly used in conjunction with these two coating types. Many underbody structural components, such as front rails on passenger cars, are made from sheet steel pre-coated with a metallic coating, e.g., galvanized or galvanneal sheet steel. Autophoretic and powder coatings are also used on underbody structural components.

*Coating type : 1. Internal Lining

Fig (4-1) internal coat

Description:
Internal coating using a two component liquid epoxy based paint.
Features:
This coating system has excellent anti-friction properties and good resistance to chemicals.

2. Fusion Bonded Epoxy (FBE) Powder Coating

Fig (4-2) Fusion Bonded Epoxy (FBE) Powder Coating

Description:
Stand alone coating system.
Features:
This coating system has adequate mechanical properties and effective anti-corrosion properties with resistance to high temperature operating service up to 120°C depending on raw materials used.

3. Dual Fusion Bonded Epoxy (D-FBE ) coating

Fig (4-3) Dual Fusion Bonded Epoxy (D-FBE ) coating

Description:
2-layer coating system composed of FBE primer (first layer), FBE topcoat (top layer).
Features:
This coating system has good mechanical properties and effective anti-corrosion properties and resistance to high temperature operating service up to 110°C or 150°C depending on raw materials used.

4. Bitumen / Asphalt Enameln (AE) Coating

Fig (4-4) Bitumen / Asphalt Enameln (AE) Coating

Description:
Multi-layer coating system composed of synthetic primer (first layer), enamel / inner wrap / enamel layer(s) (second and, if any following layers) and outer wrap layer (top layer).
Features:
This coating system has adequate mechanical properties and effective anti-corrosion properties with resistance to temperature operating service up to 90°C.

5. Three Layer Polypropylene (3LPP) Coating

Fig (4-5) Three Layer Polypropylene (3LPP) Coating

Description:
3-layer coating system composed of FBE primer (first layer), polypropylene based adhesive copolymer (second layer) and polypropylene based topcoat (top layer).
Features:
This coating system combines excellent mechanical properties and resistance to high temperature operating service up to 110°C or 150°C depending on raw materials used.

6. Three Layer Polyethylene (3LPE) Coating

Fig (4-6) Three Layer Polyethylene (3LPE) Coating

Description:
3-layer coating system composed of FBE primer (first layer), polyethylene based adhesive copolymer (second layer) and polyethylene based topcoat (top layer).
Features:
This coating system combines excellent mechanical properties and resistance to temperature operating service up to 60°C (LDPE & MDPE) or 80°C (HDPE) depending on raw materials used.

7. Concrete Weight Coating (CWC)

Fig (4-7) Concrete Weight Coating (CWC)

Description:
Weight coating system composed of cement, water, aggregates, heavy or light depending on the required density, and reinforcement.
Features:
Concrete weight coating is used to provide pipe stability on the sea bed as well as superior mechanical protection. It can be manufactured in a range of densities to suit the project specification.
8. Polyurethane Insulation Coating

Fig (4-8) Polyurethane Insulation Coating

Description:
2-layer coating system composed of FBE and syntactic polyurethane. Polymer or glass microspheres are blended to provide excellent thermal insulation properties.
Features:
Polyurethane insulation systems are designed to cover various water depths. Shallow water products (SPU) are based on a PU matrix into which polymer microspheres are blended to provide excellent thermal insulation properties. Deepwater applications (DWPU) are addressed using a range of products into which glass microspheres are blended. Both products have extensive track records.

9. Polypropylene Insulation Coating

Fig (4-9) Polypropylene Insulation Coating

Description:
Multi-layer coating system composed of polypropylene outer shield, solid polypropylene and foamed polypropylene / syntactic polypropylene.
Features:
Solid polypropylene is used as both anti-corrosion coating and thermal insulation coating where the thermal requirements are not too demanding. Another coating system is side extruded polypropylene foam which is used for thermal insulation of pipelines for water depths up to 600m. Syntactic polypropylene is used to achieve a balance between good thermal performance and thermal insulation capability for deepwater applications.

(b) Alloying the metal
to produce a more corrosion resistant alloy, e.g. stainless steel, in which ordinary steel is alloyed with chromium and nickel. Stainless steel is protected by an invisibly thin, naturally formed film of chromium oxide Cr2O3


4.3.2 Conditioning the Corrosive Environment

(a) Removal of Oxygen

By the removal of oxygen from water systems in the pH range 6.5 - 8.5 one of the components required for corrosion would be absent. The removal of oxygen could be achieved by the use of strong reducing agents e.g. sulphite. However, for open evaporative cooling systems this approach to corrosion prevention is not practical since fresh oxygen from the atmosphere will have continual access.

(b) Corrosion Inhibitors

A corrosion inhibitor is a chemical additive, which, when added to a corrosive aqueous environment, reduces the rate of metal wastage. It can function in one of the following ways:

(i) anodic inhibitors :

as the name implies an anodic inhibitor interferes with the anodic process.

Fe → Fe⁺⁺ + 2e^- eq (4-1)

If an anodic inhibitor is not present at a concentration level sufficient to block off all the anodic sites, localised attack such as pitting corrosion can become a serious problem due to the oxidising nature of the inhibitor which raises the metal potential and encourages the anodic reaction (equation 1). Anodic inhibitors are thus classified as “dangerous inhibitors”. Other examples of anodic inhibitors include orthophosphate, nitrite, ferricyanide and silicates.

(ii) cathodic inhibitors :

the major cathodic reaction in cooling systems is the reduction of oxygen.

½ O₂ + H₂O + 2e → 2OH^- eq (4-2)

There are other cathodic reactions and additives that suppress these reactions called cathodic inhibitors. They function by reducing the available area for the cathodic reaction. This is often achieved by precipitating an insoluble species onto the cathodic sites. Zinc ions are used as cathodic inhibitors because of the precipitation of Zn(OH)₂ at cathodic sites as a consequence of the localised high pH. (See reaction eq(4-2) ). Cathodic inhibitors are classed as safe because they do not cause localised corrosion.

(iii) adsorption type corrosion inhibitors:

many organic inhibitors work by an adsorption mechanism. The resultant film of chemisorbed inhibitor is then responsible for protection either by physically blocking the surface from the corrosion environment or by retarding the electrochemical processes. The main functional groups capable of forming chemisorbed bonds with metal surfaces are amino (NH₂), carboxyl (COOH), and phosphonate (PO₃H₂) although other functional groups or atoms can form co-ordinate bonds with metal surfaces.

(iv) mixed inhibitors :

because of the danger of pitting when using anodic inhibitors alone, it became common practice to incorporate a cathodic inhibitor into formulated performance was obtained by a combination of inhibitors than from the sum of the individual performances. This observation is generally referred to a ‘synergism’ and demonstrates the synergistic action which exists between zinc and chromate ions.

ALSO….. CORROSION INHIBITORS

It is well known in surface chemistry that surface reactions are strongly affected by the presence of foreign molecules. Corrosion processes, being surface reactions, can be controlled by compounds known as inhibitors which adsorb on the reacting metal surface.

The term adsorption refers to molecules attached directly to the surface, normally only one molecular layer thick, and not penetrating into the bulk of the metal itself. The technique of adding inhibitors to the environment of a metal is a well known method of controlling corrosion in many branches of technology. A corrosion inhibitor may act in a number of ways: it may restrict the rate of the anodic process or the cathodic process by simply blocking active sites on the metal surface. Alternatively it may act by increasing the potential of the metal surface so that the metal enters the passivation region where a natural oxide film forms. A further mode of action of some inhibitors is that the inhibiting compound contributes to the formation of a thin layer on the surface which stifles the corrosion process.

4.3.3 Electrochemical Control

Since corrosion is an electrochemical process its progress may be studied by measuring the changes which occur in metal potential with time or with applied electrical currents. Conversely, the rate of corrosion reactions may be controlled by passing anodic or cathodic currents into the metal. If, for example, electrons are passed into the metal and reach the metal/electrolyte interface (a cathodic current) the anodic reaction will be stifled while the cathodic reaction rate increases. This process is called cathodic protection and can only be applied if there is a suitable conducting medium such as earth or water through which a current can flow to the metal to be protected In most soils or natural waters corrosion of steel is prevented if the potential of the metal surface is lowered by 300 or 400 mV. Cathodic protection may be achieved by using a DC power supply (impressed current) or by obtaining electrons from the anodic dissolution of a metal low in the galvanic series such as aluminium, zinc or magnesium (sacrificial anodes). Similar protection is obtained when steel is coated with a layer of zinc. Even at scratches or cut edges where some bare metal is exposed the zinc is able to pass protective current through the thin layer of surface moisture.

In certain chemical environments it is sometimes possible to achieve anodic protection, passing a current which takes electrons out of the metal and raises its potential. Initially this stimulates anodic corrosion, but in favourable circumstances this will be followed by the formation of a protective oxidised passive surface film.

Also …..

Cathodic protection prevents corrosion by converting all of the anodic (active) sites on the metal surface to cathodic (passive) sites by supplying electrical current (or free electrons) from an alternate source.

Usually this takes the form of galvanic anodes which are more active than steel. This practice is also referred to as a sacrificial system, since the galvanic anodes sacrifice themselves to protect the structural steel or pipeline from corrosion. In the case of aluminum anodes, the reaction at the aluminum surface is:

4Al => 4AL⁺⁺⁺ + 12 e^- eq(4-3)

and at the steel surface,

3O₂ + 12e^- + 6H₂0 => 12OH^- eq(4-4)

(Oxygen gas converted to oxygen ions which combine with water to form hydroxyl ions)

As long as the current (free electrons) is arriving at the cathode (steel) faster than oxygen is arriving, no corrosion occurs.

Fig (4-10) Sacrificial Anode CP System in Seawater

· Anodic protection

Fontana and Greene’ state that ‘anodic protection can be classed as one of the most significant advances in the entire history of corrosion science’, but point out that its adoption in corrosion engineering practice is likely to be slow. Anodic protection may be described as a method of reducing the corrosion rate of immersed metals and alloys by controlled anodic polarisation, which induces passivity. Therefore, it can be applied only to those metals and alloys that show passivity when in contact with an appropriate electrolyte. This decrease in corrosion increases the life of components/plant as well as reducing the contamination of the liquid, so is particularly beneficial in the manufacture, storage and transport of chemicals such as acids. Edeleanu first demonstrated the feasibility of anodic protection and also tested it on small-scale stainless-steel boilers used for sulphuric acid solutions .

This was probably the first industrial application, although other experimental work had been carried out elsewhere. Fortunately electrochemical tests in the laboratory can give an accurate assessment of the corrosion behaviour, and the operating parameters for a specific anodic protection system can be obtained .

· Finally the anodic protection is :

• suitable for active-passive alloys (e.g. stainless steel, nickel alloys, titanium)

• requires a broad potential range for passivity

• need sizable/expensive electrical equipment

• risky if potential “slips” into the active/pitting region

• used often for very aggressive solutions when other methods fail, e.g. for protection of tanks storing of strong acids (e.g. sulphuric, phosphoric, nitric)

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Material selection

4.2 Material selection

4.2.1 Alloy steels

The corrosion resistance of steels can be markedly improved by adding other metals to produce alloys. The most resistant of the common steel alloys is stainless steel. It is a good deal more expensive than ordinary steel and, although widely used in process plant, is employed to only a limited extent in structures, mainly for fasteners in particularly aggressive situations and sometimes for bearings. It is more widely used on buildings for cladding, balustrades, doors, etc. Although there are a number of different groups within the overall classification of stainless steel, the one most commonly used in buildings an structures is austenitic stainless steel, so described because of its metallurgical structure. In fact, steels with 12% or more of chromium fall into the category of stainless steels but the common austenitic types contain over 30% of alloying elements, 18–20% chromium, 8–10% nickel and about 3% molybdenum.The other group of alloy steels that have been used for structures and buildings are much lower in alloy content, only about 2–3%. These are called ‘weathering steels’, the best known of which is the US Steel Corporation version ‘COR TEN’, also produced under licence in other countries. Unlike stainless steels they have been used for structural members as well as cladding for buildings.

4.2.2 Stainless steels

These steels owe their corrosion resistance to the formation of a passive surface oxide film, basically Cr2O3.

• Corrosion characteristics of stainless steels

The austenitic stainless steels are virtually uncorroded when freely exposed in most atmospheric environments. The 304 series, without molybdenum additions, may exhibit rust staining arising from slight pitting but the actual loss of steel by corrosion is negligible. The 304 steels are attacked to a greater extent in marine atmospheres because of the presence of chlorides, and this may lead to a rust-stained appearance but again produces little loss of metal. The 315 and 316 steels perform well even in marine atmospheres and often under immersed conditions. However, in some immersed situations corrosion can occur, particularly in stagnant conditions where marine growths can form. Such organisms shield the steel from oxygen so that breakdown of the passive film is not repaired. Any area where the film cannot be repaired is a potential site for pitting. Such situations as overlaps and crevices may provide conditions where pitting may occur. This is not likely to be serious in most atmospheric conditions but may be more severe under immersed situations. Pitting occurs to a much greater extent on stainless steels than on carbon steels.

This arises from the presence of the very protective film, which becomes cathodic to any small breaks where local corrosion occurs. In the presence of an electrolyte, the corroding area, i.e. the anodic part of the cell, is in contact with a large cathodic area, which intensifies the local corrosion. Since the passive film is very adherent at the edge of the local anodic area, corrosion tends not to spread sideways but rather to penetrate into the alloy, i.e. to cause pitting. Such pitting can be serious if comparatively thin sheet material is used as a pipe for transporting liquids, because eventually the steel is perforated by the pitting, allowing escape of the liquid. In most situations where stainless steel is used for structures, this is not such a serious problem, but care should be taken with the design of stainless steel fabrications, particularly where they are exposed to chlorides which are the species most likely to cause pitting. Marine situations are obviously affected by chlorides, but attention should also be paid to the effects of deicing salts when stainless steel is used on bridges. These steels are often used as components for structures and buildings but they are also used for architectural panels. Generally, 316-type material is employed for this purpose and care must be exercised during construction to ensure that mortars and cements do not come into contact with the panels. Problems of pitting can occur, particularly with chloride containing concretes, especially if they are allowed to set and are not immediately removed. In cities and large towns where stainless steel may be used for cladding, the accumulations of dirt, particularly if not exposed to rainfall, can lead to local breakdown of the passive film and it is advantageous to wash the steel down regularly.


4.2.1.2 Low-alloy weathering steels

In the early 1970s a large number of bridges as well as other structures and buildings were constructed from these steels, the best known of which was called ‘CORTEN’, mainly in the USA but also in many other countries, including the United Kingdom. Small additions of alloying elements such as copper, nickel, chromium and somewhat higher amounts of silica and phosphorous than in ordinary steels resulted in an alloy content of only 2–3%. This had the effect of reducing the corrosion rate in air compared with that of unalloyed steel. Furthermore, although initially weathering steels rusted in a similar manner to ordinary steels, after a period of some months the rust became darker and more adherent than conventional rust. Considerable test work on small panels throughout the world confirmed the advantages of these steels provided they were freely exposed in air at inland sites. Their performance compared with ordinary steel showed less improvement when exposed close to the sea, and if they were immersed in water or buried in soil their corrosion rate was similar to that of ordinary steel.
In practice there have been disappointments with the use of these steels; their appearance is variable depending upon orientation and the loose powdery rust is a nuisance and can stain adjacent areas.
Although there probably is a place for weathering steels in certain situations, the design of structures and buildings must take into account the corrosion properties of the steels. This has not always been done in a satisfactory way. A paper by Tinklenberg and Culp3 sums up what is probably a fairly representative view of many bridge authorities in the USA.
Theauthors say:
in 1977, a comprehensive evaluation of weathering steel was started. This investigation identified a number of problem areas. These included salt contamination, crevice corrosion, pitting, millscale, accumulation of debris, the capillarity of the rust by-products and the potential of corrosion fatigue. When it was determined that these structures had to be painted and that other equal strength steels were available at a lower cost, the initial reasons for selecting weathering steel were no longer valid .

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corrosion protection

4.1 FACTORS THAT CONTROL THE CORROSION RATE

Certain factors can tend to accelerate the action of a corrosion cell .

These include :

(a) Establishment of well-defined locations on the surface for the anodic and cathodic reactions. This concentrates the damage on small areas where it may have more serious effects, this being described as “local cell action”. Such effects can occur when metals of differing electrochemical properties are placed in contact, giving a “galvanic couple”. Galvanic effects may be predicted by means of a study of the Galvanic Series which is a list of metals and alloys placed in order of their potentials in the corrosive environment, such as sea water. Metals having a more positive (noble) potential will tend to extract electrons from a metal which is in a more negative (base) position in the series and hence accelerate its corrosion when in contact with it. The Galvanic Series should not be confused with the Electrochemical Series, which lists the potentials only of pure metals in equilibrium with standard solutions of their ions.

Galvanic effects can occur on metallic surfaces which contain more than one phase, so that “local cells” are set up on the heterogeneous surface. Localised corrosion cells can also be set up on surfaces where the metal is in a varying condition of stress, where rust, dirt or crevices cause differential access of air, where temperature variations occur, or where fluid flow is not uniform.

(b) Stimulation of the anodic or cathodic reaction. Aggressive ions such as chloride tend to prevent the formation of protective oxide films on the metal surface and thus increase corrosion. Sodium chloride is encountered in marine conditions and is spread on roads in winter for de-icing.

Quite small concentrations of sulphur dioxide released into the atmosphere by the combustion of fuels can dissolve in the invisibly thin surface film of moisture which is usually present on metallic surfaces when the relative humidity is over 60-70%. The acidic electrolyte that is formed under these conditions seems to be capable of stimulating both the anodic and the cathodic reactions. In practical terms it is not usually possible to eliminate completely all corrosion damage to metals used for the construction of industrial plant. The rate at which attack is of prime importance is usually expressed in one of two ways:

(1) Weight loss per unit area per unit time, usually mdd (milligrams per square decimeter per day)

(2) A rate of penetration, i.e. the thickness of metal lost. This may be expressed in American units, mpy (mils per year, a mil being a thousandth of an inch) or in metric units, mmpy (millimetres per year). Taking as an example the corrosion of heat exchanger tubes in industrial cooling water a typical corrosion rate in untreated water would be 40-50 mpy (210-260 mdd); the use of a corrosion inhibitor could reduce this to less than 5 mpy (26 mdd). The mild steel tubing used in heat exchangers is a maximum of 200 thousandths of an inch thick, thus with corrosion rates of 40-50 mpy in untreated water, severe problems might be expected within four or five years. If suitable water treatment with corrosion inhibitors is used a life of at least twenty years might be expected. This, of course, is ignoring the fact that at some time before the metal corrodes away the tubing may have thinned to a point where its required mechanical strength is not attained. When designing equipment for a certain service life engineers often add a “corrosion allowance” to the metal thickness, permitting a certain amount of thinning before serious weakening occurs. In a cooling water system the factors influencing the rate of attack are:

(a) the condition of the metal surface Corrosion debris and other deposits - corrosion under the deposits, with a possibility of pitting (severe attack in small spots)

(b) the nature of the environment pH - in the range of 4-10 corrosion rate is fairly independent of pH, but it increases rapidly when the pH falls below 4. Oxygen content - increase in oxygen concentration usually gives an increase in corrosion rate. Flow rate - increased water flow increased oxygen access to the surface and removes protective surface films, so usually increases corrosion, but can sometimes improve access for corrosion inhibiting reactants. Water type - very important, in general low corrosion rates are found with scale-forming (hard) waters. Aggressive ions which accelerate corrosion are Cl^- , SO₄^2- but quite complex interactions may occur between the various dissolved species in natural waters. number of such methods have been developed, and they are set out in Table 1. The table shows a variety of different concepts by which the surface reaction rate can be reduced. Each of these has given rise to a number of technologies, the majority of which are represented in New Zealand industry. In some cases these industries are on a very large scale. For example paint manufacture is a major chemical industry which consumes large quantities of solvents, resins and pigments. Most paint products in New Zealand are used in corrosion protection. Other major industries involved in corrosion control include electroplating, anodising, galvanising and the production of corrosion resistant alloys. In this article we will concentrate on two important methods of corrosion control used in New Zealand industry, namely cathodic protection and chemical inhibitors. Other types of corrosion control technology, such as electroplating and surface coatings, are covered elsewhere.

Table (4-1) Corrosion protection techniques

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Environment Effects

3.1 Atmospheric Corrosion

Atmospheric corrosion can be defined as the corrosion of materials exposed to air and its pollutants, rather than immersed in a liquid. Atmospheric corrosion can further be classified into dry, damp, and wet categories. This chapter deals only with the damp and wet cases, which are respectively associated with corrosion in the presence of microscopic electrolyte (or “moisture”) films and visible electrolyte layers on the surface. The damp moisture films are created at a certain critical humidity level (largely by the adsorption of water molecules), while the wet films are associated with dew, ocean spray, rainwater, and other forms of water splashing. By its very nature, atmospheric corrosion has been reported to account for more failures in terms of cost and tonnage than any other factor

3.1.1 Types of atmospheres and environments

The severity of atmospheric corrosion tends to vary significantly among different locations, and, historically, it has been customary to classify environments as rural, urban, industrial, marine, or combinations of these. These types of atmosphere have been described as follows:

· Rural. This type of atmosphere is generally the least corrosive and normally does not contain chemical pollutants, but does contain organic and inorganic particulates. The principal corrodents are moisture, oxygen, and carbon dioxide. Arid and tropical types are special extreme cases in the rural category.

· Urban. This type of atmosphere is similar to the rural type in that there is little industrial activity. Additional contaminants are of the SOx and NOx variety, from motor vehicle and domestic fuel emissions.

· Industrial. These atmospheres are associated with heavy industrial processing facilities and can contain concentrations of sulfur dioxide, chlorides, phosphates, and nitrates.

· Marine. Fine windswept chloride particles that get deposited on surfaces characterize this type of atmosphere. Marine atmospheres are usually highly corrosive, and the corrosivity tends to be significantly dependent on wind direction, wind speed, and distance from the coast. It should be noted that an equivalently corrosive environment is created by the use of deicing salts on the roads of many cold regions of the planet.

3.1.2 Important practical variables in atmospheric corrosion

3.1.2.1 Time of wetness :

From the above theory, it should be apparent that the time of wetness (presence of electrolyte on the corroding surface) is a key parameter, directly determining the duration of the electrochemical corrosion processes. This variable is a complex one, since all the means of formation and evaporation of an electrolytic solution on a metal surface must be considered. The time of wetness is obviously strongly dependent on the critical relative humidity. Apart from the primary critical humidity, associated with clean surfaces, secondary and even tertiary critical humidity levels may be created by hygroscopic corrosion products and capillary condensation of moisture in corrosion products, respectively. A capillary condensation mechanism may also account for electrolyte formation in microscopic surface cracks and the metal surface–dust particle interface. Other sources of surface electrolyte include chemical condensation (by chlorides, sulfates, and carbonates), adsorbed molecular water layers, and direct moisture precipitation (ocean spray, dew,rain). The effects of rain on atmospheric corrosion damage are somewhat ambiguous. While providing electrolyte for corrosion reactions, rain can act in a beneficial manner by washing away or diluting harmful corrosive surface species.

3.1.2.2 Sulfur dioxide :

Sulfur dioxide, a product of the combustion of sulfur containing fossil fuels, plays an important role in atmospheric corrosion in urban and industrial atmospheres. It is adsorbed on metal surfaces, has a high solubility in water, and tends to form sulfuric acid in the presence of surface moisture films. Sulfate ions are formed in the surface moisture layer by the oxidation of sulfur dioxide in accordance

SO₂ + O₂ + 2e^- → SO₄^2- eq (3-1)

The required electrons are thought to originate from the anodic dissolution reaction and from the oxidation of ferrous to ferric ions. It is the formation of sulfate ions that is considered to be the main corrosion accelerating effect from sulfur dioxide. For iron and steel, the presence of these sulfate ions ultimately leads to the formation of iron sulfate (FeSO₄). Iron sulfate is known to be a corrosion product component in industrial atmospheres and is mainly found in layers at the metal surface.

The iron sulfate is hydrolyzed by the reaction expressed by

FeSO₄ + 2H₂O → FeOOH + SO₄ ^2- + 3H⁺ + e^- eq (3-2)

The corrosion-stimulating sulfate ions are liberated by this reaction, leading to an autocatalytic type of attack on iron.8 – 10 The acidification of the electrolyte could arguably also lead to accelerated corrosion rates, but this effect is likely to be of secondary importance because of the buffering effects of hydroxide and oxide corrosion products. In nonferrous materials such as zinc, sulfate ions also stimulate corrosion, but the autocatalytic corrosion mechanism is not easily established. Corroding zinc tends to be covered by stable zinc oxides and hydroxides, and this protective covering is only gradually destroyed at its interface with the atmosphere. In moderately corrosive atmospheres, sulfates present in zinc corrosion products tend to

be bound relatively strongly, with limited water solubility. At very high levels of sulfur dioxide, dissolution of protective layers and the formation of more soluble corrosion products is associated with higher corrosion rates.

3.1.2.3 Chlorides :

Atmospheric salinity distinctly increases atmospheric corrosion rates. Apart from the enhanced surface electrolyte formation by hygroscopic salts such as NaCl and MgCl₂, direct participation of chloride ions in the electrochemical corrosion reactions is also likely. In ferrous metals, chloride anions are known to compete with hydroxyl ions to combine with ferrous cations produced in the anodic reaction. In the case of hydroxyl ions, stable passivating species tend to be produced. In contrast, iron-chloride complexes tend to be unstable (soluble), resulting in further stimulation of corrosive attack. On this basis, metals such as zinc and copper, whose chloride salts tend to be less soluble than those of iron, should be less prone to chloride-induced corrosion damage, and this is consistent with practical experience. Other atmospheric contaminants. Hydrogen sulfide, hydrogen chloride, and chlorine present in the atmosphere can intensify atmospheric corrosion damage, but they represent special cases of atmospheric corrosion that are invariably related to industrial emissions in specific microclimates. Hydrogen sulfide is known to be extremely corrosive to most metals / alloys, and the corrosive effects of gaseous chlorine and hydrogen chloride in the presence of moisture tend to be stronger than those of “chloride salt” anions because of the acidic character of the former

3.1.2.4 Nitrogen compounds :

Nitrogen compounds, in the form of NOx , also tend to accelerate atmospheric attack. NOx emission, largely from combustion processes, has been reported to have increased relative to SO₂ levels. However, measured deposition rates of these nitrogen compounds have been significantly lower than those for SO₂, which probably accounts for the generally lower importance assigned to these. Until recently, the effects of ozone (O₃) had been largely neglected in atmospheric corrosion research. It has been reported that the presence of ozone in the atmosphere may lead to an increase in the sulfur dioxide deposition rate. While the accelerating effect of ozone on zinc corrosion appears to be very limited, both aluminum and copper have been noted to undergo distinctly accelerated attack in its presence. The deposition of solid matter from the atmosphere can have a significant effect on atmospheric corrosion rates, particularly in the initial stages. Such deposits can stimulate atmospheric attack by three mechanisms:

· Reduction in the critical humidity levels by hygroscopic action

· The provision of anions, stimulating metal dissolution

· Microgalvanic effects by deposits more noble than the corroding metal .

3.1.2. 5 Temperature :

The effect of temperature on atmospheric corrosion rates is also quite complex. An increase in temperature will tend to stimulate corrosive attack by increasing the rate of electrochemical reactions and diffusion processes. For a constant humidity level, an increase in temperature would lead to a higher corrosion rate. Raising the temperature will, however, generally lead to a decrease in relative humidity and more rapid evaporation of surface electrolyte. When the time of wetness is reduced in this manner, the overall corrosion rate tends to diminish. For closed air spaces, such as indoor atmospheres, it has been pointed out that the increase in relative humidity associated with a drop in temperature has an overriding effect on corrosion rate. This implies that simple air conditioning that decreases the temperature without additional dehumidification will accelerate atmospheric corrosion damage. At temperatures below freezing, where the electrolyte film

3.2 Corrosion By Water

Nearly all corrosion problems which occur in oilfield production operations are due to the presence of water. In order to corrode, the metal surface must be in contact with a water phase. For example, if a well produces at a high oil-to water ratio, very little corrosion is likely to occur because the water is mixed with oil as an oil-external emulsion. On the other hand, in low oil-to-water ratio wells, corrosion occurs because free water contacts the metal surface.

Corrosion in the presence of water depends on electrochemical processes. Electric current flows and there must be a driving force and a complete electrical circuit.

3.2.1 Effect OF Electrolyte Composition

There are two aspects to the effects of electrolyte composition on the corrosion circuit. The first is the conductivity of the electrolyte and the effect of electrolyte on the base corrosion potential of the system. The second has to do with the presence or absence of oxidizing agents which are necessary for the cathodic portion of the corrosion cell. The anodic reaction cannot occur in the absence of a corresponding reaction at the cathode, regardless of the conductivity of the cathode.

3.2.1.1 Conductivity

The electrical resistance of typical electrolytes is usually much higher than that of metal, therefore the resistance of the electrolyte will normally predominate in the corrosion cell reaction. The more conductive the electrolyte, the easier current can flow and the faster corrosion will occur. The amount of metal that dissolves is directly proportional to the amount of current flow between anode and cathode. For iron, one amp of current flowing for one year will result in the loss of 20 pounds (9.1 kg) of metal. It is important to remember that other factors will also have an impact on the corrosivity of the electrolyte, the conductivity only determining the ease at which corrosion currents are able to flow from anode to cathode.

3.2.1.2 Hydrogen Ion Concentration (pH)

The pH of water is the negative logarithm of the hydrogen ion concentration:

pH = - log (H⁺) eq (3-3)

The greater the concentration of hydrogen ions, the more acid the solution and the lower the pH value. Hydrogen ions (H+) make a solution acidic and, therefore, force the pH towards zero. Hydroxyl ions (OH ‾ ) make a solution basic or alkaline and force the pH towards 14.

The following lists the pH ranges for acidic, neutral, and alkaline conditions:

Table ( 3-1) pH ranges for acidic, neutral, and alkaline conditions [11]

The corrosion rate of steel usually increases as the pH of the water decreases, although extremely high pH solutions can also be corrosive

Fig (3-1) Corrosion Rate of Steel vs pH [11]

The actual variation of corrosion rate with pH is dependent on the composition of the electrolyte

3.2.1.3 Dissolved Gases

Oxygen, carbon dioxide, or hydrogen sulphide gases, when dissolved in water, increase its corrosivity. Dissolved gases are the primary cause of most corrosion problems in oil and gas production. The following paragraphs discuss each gas independently, but it is important to note that corrosion rates are also greatly influenced by physical variables such as temperature, pressure and velocity. Similarly, the Figures referred in these paragraphs are for specific conditions and are only intended to reflect the relative corrosion tendencies of each.

3.2.1.3.1 Oxygen

Dissolved oxygen can cause severe corrosion at very low concentrations (less than 100 ppb or 0.1 ppm) and if either or both CO2 and/or H2S are present, it further increases their corrosivity.

Fig (3-2) is a composite graph from results of three different studies showing corrosion rates as a function of oxygen concentration [11]

The solubility of oxygen in water is a function of pressure, temperature, and chloride content. Although it is not usually present in produced water, it is often introduced into oilfield water handling systems through failures to maintain oxygen free gas blankets on water handling vessels, vacuums created by positive displacement pumps or separator dump valves and/or exposure to the atmosphere. Water from lakes, streams, fresh water aquifers, rain or oceans usually will be oxygen saturated. Oxygen is more soluble at high pressures and lower temperatures and is less soluble in salt water than in fresh water.

Oxygen accelerates corrosion under most circumstances because it is a strong and rapid oxidising agent in cathodic reactions. It will easily combine with electrons at the cathode and allow the corrosion reactions to proceed at a rate limited by the rate at which oxygen can diffuse to the cathode

O₂+ 4H⁺ + 4e^- → 2H₂O (Acidic Solutions) eq (3-4)

OR

O₂+ 2H₂O + 4e^- → 4OH^- (Neutral or Alkaline Solutions) eq (3-5)

3.2.1.3.2 Carbon Dioxide

When carbon dioxide dissolves in water, it forms carbonic acid, decreases the pH of the water, and increases its corrosivity. Corrosion in the presence of dissolved CO₂ is referred to as sweet corrosion. There are numerous

intermediate reactions which may be summarised as:

CO₂ + H₂O → H₂CO₃ (Carbonic acid) eq (3-6)

Fe + H₂CO₃ → FeCO₃ + H₂ (Iron Carbonate) eq (3-7)

Factors governing the solubility of carbon dioxide are pressure, temperature, and composition of the water. Increased pressure, reduced temperature, or reduced water salinity each increase CO₂ solubility which lowers pH. Many dissolved minerals buffer the water, thus minimising the effects of the above changes on pH reduction.

Fig (3-3) CO2 General Corrosion Rate of Steel [11]

Partial pressure of carbon dioxide can be used as a yardstick to predict the corrosiveness of a system. The partial pressure of carbon dioxide can be determined by the formula:

CO₂ partial pressure (psia pp) = Total pressure of gas (psia) x Mole fraction of

CO₂ in gas eq(3-7)

In general, field experience indicates that, in the presence of an electrolyte, a partial pressure above about 30 psia may cause severe corrosion rates; between 7 psia and 30 psia may cause high corrosion rates and, less than 7 psia can still cause low to moderate corrosion rates.

2.1.3.3 Hydrogen Sulphide

Hydrogen sulphide is soluble in water at pressures and temperatures common in oilfield operations and, when dissolved, behaves as a weak acid and usually causes pitting. Attack due to the presence of dissolved hydrogen sulphides is referred to as sour corrosion.

The general corrosion reaction of steel is:

H₂S + Fe → FeS + H₂ eq (3-8)

The iron sulphide produced generally adheres to the surface as a black scale and is cathodic to the steel that, in the presence of water, causes local severe corrosion in the form of deep pitting. However, in some instances, a thin iron sulphide scale may be relatively impermeable and actually slow down the corrosion reaction if erosion or some other mechanism does not remove the scale.

Hydrogen sulphide can be generated by sulphate reducing bacteria (SRB). These bacteria contribute to corrosion by their ability to flourish in the absence of oxygen and their ability to change sulphate ions into hydrogen sulphide.

The anaerobic conditions under a colony constitute a differential aeration cell with the bulk of the electrolyte, whereas their ability to produce hydrogen sulphide can cause severe localized corrosion.

Under certain pressure conditions, the hydrogen produced by the corrosion reaction can diffuse into the metallic lattice to cause embrittlement and subsequent cracking of susceptible metals.

3.2.1.3.4 Chloride Ions

The most common electrolyte in oil production is water, and one of the most common ions in solution is the chloride ion. The chloride ion and its concentration has a major effect on the corrosion reactions as noted below:

· An increase in concentration of chlorides increases the conductivity of the solution and, therefore, allows corrosion currents to occur more rapidly.

· Chloride anions (negatively charged ions) tend to react very easily with cations (e.g. Fe⁺²) going into solution at the corrosion cell anode. These reactions, therefore, reduce polarization by allowing more cations to come into solution that increases the conductivity of the electrolyte.

· Although not a weight loss type corrosion, increased concentration of chloride ions increases the susceptibility of austenitic stainless steels to pit and crack. and also refer to the CIMS Metallurgy and Metallic Material Selection Guidelines.

3.2.2 PHYSICAL VARIABLES 

The variables of temperature, pressure, and velocity need to be accounted for when designing and implementing a corrosion control program. Correct application inhibitors and cathodic protection as corrosion control methods are very dependent on these variables. Temperature and pressure are interrelated, and the corrosivity of a system is further influenced by velocity.

3.2.2 .1 Temperature

Like most chemical reactions, corrosion rates generally increase with temperature. For example, in a system open to the atmosphere, the corrosion rate generally increases with increasing temperature until the concentration of dissolved gases decreases. In a closed system, this is not necessarily the case. In addition, many metallic alloys have minimum temperature limitations to prevent H₂S service related cracking or other toughness problems.

3.2.2.2 Pressure

Pressure also affects the rates of corrosion reactions. More gas goes into solution as the pressure increases, which may, depending on the dissolved gas, increase the corrosivity of the solution. The partial pressure of CO₂ or H₂S in a system is calculated as follows

partial pressure (psia pp) = Total absolute pressure (psia) x mol % CO₂ or H₂S eq (3-9)

3.2.2.3 Velocity

Velocity has a significant effect on corrosion rates. Stagnant or low velocity fluids usually give low general corrosion rates, but pitting rates may be high. Corrosion rates generally increase with increasing velocity due to the depolarising effect on the cathode. High velocities and the presence of suspended solids or gas bubbles can lead to erosion corrosion, impingement, or cavitation. On the other extreme, oil, gas, or multi-phase pipelines operating at low velocities can result in corrosion along the bottom of a pipeline. The low flow condition is referred to as stratified or laminar flow which can be modeled using commercially available computer programs.

3.2.2.4 Erosion/Velocity

Any fluid, gas, or multiphase pipeline or piping system should be operated below its calculated erosional velocity to prevent flow enhanced corrosion (erosion corrosion). API RP 14E, “Design and Installation of Offshore Production Platform Piping Systems,” provides the following equation to calculate the erosional velocity limit for any oil or gas piping:

V_e = C ⁄ (ρm)^{1⁄ 2} eq(3-10)

*where:

Ve = erosional flow velocity (ft/sec)

ρm = fluid/gas mix density (lb/ft3)

C = empirical constant, where

C = 100, per API RP 14E for carbon steels or,

C = 160+ for corrosion resistant alloys

3.3 Soil in the Corrosion Process

Introduction

Soil has been defined in many ways, often depending upon the particular interests of the person proposing the definition. In discussion of the soil as an environmental factor in corrosion, no strict definitions or limitations will be applied; rather, the complex interaction of all earthen materials will come within the scope of the discussion. It is obvious only a general approach to the topic can be given, and no attempt will be made to give full and detailed information on any single facet of the topic. Soil is distinguished by the complex nature of its composition and of its interaction with other environmental factors. No two soils are exactly alike, and extremes of structure, composition and corrosive activity are found in different soils. Climatic factors of rainfall, temperature, air movement and sunlight can cause marked alterations in soil properties which relate directly to the rates at which corrosion will take place on metals buried in these soils.

3.3.1 The Corrosion Process in Soil

Although the soil as a corrosive environment is probably of greater complexity than any other environment, it is possible to make some generalizations regarding soil types and corrosion. It is necessary to emphasise that corrosion in soils is extremely variable and can range from the rapid to the negligible.

Fig (3-4) corrosion by soil

This can be illustrated by the fact that buried pipes have become perforated within one year, while archaeological specimens of ancient iron have probably remained in the soil for hundreds of years without significant attack. Corrosion in soil is aqueous, and the mechanism is electrochemical, but the conditions in the soil can range from ‘atmospheric’ to completely immersed. Which conditions prevail depends on the compactness of the soil and the water or moisture content. Moisture retained within a soil under field dry conditions is largely held within the capillaries and pores of the soil. Soil moisture is extremely significant in this connection, and a dry sandy soil will, in general, be less corrosive than a wet clay. Although the mechanism will be essentially electrochemical, there are many characteristic features of soil as a corrosive environment which will be considered subsequently; it can, however, be stated here that the actual corrosiveness of a soil will depend upon an interaction between rainfall, climateand soil reaction.

A characteristic feature of the soil is its heterogeneity. Thus variation in soil composition or structure can result in different environments acting on different parts of the same metal surface, and this can give rise to differing electrical potentials at the metal/soil interface. This will result in the establishment of predominantly cathodic or predominantly anodic areas, and the consequent passage of charge through the metal and through the soil.

Differences in oxygen concentration (differential aeration), or differences in acidity or salt concentrations may thus give rise to corrosion cells. The distance of the separation of the anodic and cathodic areas can range from very small to miles (‘long-line’ corrosion). The conductivity of the soil is important as it is evident from the electrochemical mechanism of corrosion that this can be rate-controlling; a high conductivity will be conducive of a high corrosion rate. In addition, the conductivity of the soil is important for ‘stray-current corrosion’

3.3.2 Properties of Soils Related to Corrosion

§ Soil Texture and Structure

Soils are commonly named and classified according to the general size range of their particulate matter. Thus sandy, silt and clay types derive their names from the predominant size range of inorganic constituents. Particles between 0.07 and about 2 mm are classed as sands. Silt particles range from 0.005mm to 0.07, and clay particle size ranges from 0.005mm mean diameter down to colloidal matter. The proportion of the three size groups will determine many of the properties of the soil. for various proportions of sand, silt and clay. Since soils contain organic matter, moisture, gases and living organisms as well as mineral particles, it is apparent that the relative size range does not determine the whole nature of the soil structure. In fact most soils consist of aggregates of particles within a matrix of organic and inorganic colloidal matter rather than separate individual particles. This aggregation gives acrumb-like structure to the soil, and leads to friability, more ready penetration of moisture, greater aeration, less erosion by water and wind, and generally greater biological activity. The loss of the aggregated structure can occur as the result of mechanical action, or by chemical alteration such as excess alkali accumulation. Destruction of the structure or ‘puddling’ greatly alters the physical nature of the soil. Mention should be made of the soil profile (section through soil showing various layers) because it is important to recognise that the soil’s surface

§ Aeration and Oxygen Diffusion

The pore space of a soil may contain either water or a gaseous atmosphere. Thus the aeration of a soil is directly related to the amount of pore space present and to the water content. Soils of fine texture due to a high clay content contain more closely packed particles and have less pore capacity for gaseous diffusion than an open-type soil such as sand. Oxygen content of soil atmosphere is of special interest in corrosion. It is generally assumed that the gases of the upper layers of soil are similar in composition to the atmosphere above the soil, except for a higher carbon dioxide content. Relatively few data are available showing oxygen content of soils at depths of interest to the corrosion engineer. Judging by the fact that plant roots require oxygen to penetrate a soil, however, it may be assumed that soil gases at depths of 6 m or more contain significant amounts of oxygen. Diffusion of gases into soil is enhanced by a number of climatic factors.

Temperature changes from day to night conditions cause expansion and contraction of the surface-soil gases. Variation in barometric pressure has a bellows-like effect on gaseous diffusion. To illustrate the magnitude of this diffusion rate on a large scale, it may be recalled that air within the more than 43 km of underground passages of the Carlsbad Caverns in New Mexico undergoes a complete change each day, despite the fact that the single opening of these caverns to the surface is only a metre or so in diameter. Biological activity within the soil tends to decrease the oxygen content and replace the oxygen with gases from metabolic activity, such as carbon dioxide. Most biological activity occurs in the upper 150 mm of soil, and it is in this region that diffusion would be most rapid. Factors which tend to increase microbial respiration, such as the addition of large amounts of readily decomposed organic matter, or factors which decrease diffusion rates (water saturation) will lead to development of anaerobic conditions within the soil.

§ Water Relations

No corrosion occurs in a completely dry environment. In soil, water is needed for ionisation of the oxidised state at the metal surface. Water is also needed for ionisation of soil electrolytes, thus completing the circuit for flow of a current maintaining corrosive activity. Apart from its participation in the fundamental corrosion process, water markedly influences most of the other factors relating to corrosion in soils. Its ro1e in weathering and soil genesis has already been mentioned.

3.3.3 Types of Soil Moisture

1. Free ground water. At some depth below the surface, water is constantly present. This distance to the water table may vary from a few metres to hundreds of metres, depending upon the geological formations present. Only a small amount of the metal used in underground service is present in the ground water zone. Such structures as well casings and under-river pipelines are surrounded by ground water. The corrosion conditions in such a situation are essentially those of an aqueous environment.

2. Gravitational water. Water entering soil at the surface from rainfall or some other source moves downward. This gravitational water will flow at a rate governed largely by the physical structure regulating the pore space at various zones in the soil profile. An impervious layer of clay, a ‘puddled’ soil, or other layers of material resistant to water passage may act as an effective barrier to the gravitational water and cause zones of water accumulation and saturation. This is often the situation in highland swamp and bog formation. Usually gravitational water percolates rapidly to the level of the permanent ground water.

3. Capillaty water. Most soils contain considerable amounts of water held in the capillary spaces of the silt and clay particles. The actual amount present depends upon the soil type and weather conditions. Capillary moisture represents the important reservoir of water in soil which supplies the needs of plants and animals living in or on the soil. Only a portion of capillary water is available to plants. ‘Moisture-holding capacity’ of a soil is a term applied to the ability of a soil to hold water present in the form of capillary water. It is obvious that the moisture-holding capacity of a clay is much greater than that of a sandy type soil. Likewise, the degree of corrosion occurring in soil will be related to its moisture-holding capacity, although the complexities of the relationships do not allow any quantitative or predictive applications of the present state of knowledge.

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