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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