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.