NACE Resource Center
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Stainless Steel Corrosion
There are five main types of stainless steel: ferritic, martensitic, austenitic, precipitation hardening and duplex. The ferritic and martensitic grades are so named because of their crystal structures. Both are iron-chromium-based alloys and were the type of stainless steel first developed in the early 1900’s. The ferritic and martensitic stainless steels are magnetic. The martensitic stainless steels can be hardened by a heat treatment similar to that used to harden ordinary steel, namely, heating to a high temperature, quenching, then reheating to an intermediate temperature (tempering) to achieve the desired balance of hardness and ductility.
Stainless and heat resisting steels possess unusual resistance to attack by corrosive media at atmospheric and elevated temperatures, and are produced to cover a wide range of mechanical and physical properties for particular applications.
Along with iron and chromium, all stainless steels contain some carbon. It is difficult to get much less than about 0.03 % and sometimes carbon is deliberately added up to 1.00% or more. The more carbon there is, the more chromium must be used, because carbon can take from the alloy about seventeen times its own weight of chromium to form carbides. Chromium carbide is of little use for ing resist corrosion. The carbon, of course, is added for the same purpose as in ordinary steels to make the alloy stronger.
Stainless steels are mainly used in wet environments. With increasing chromium and molybdenum contents, the steels become increasingly resistant to aggressive solutions. The higher nickel content reduces the risk of SCC. Austenitic steels are more or less resistant to general corrosion, crevice corrosion and pitting, depending on the quantity of alloying elements. Resistance to pitting and crevice corrosion is very important if the steel is to be used in chloride containing environments. Resistance to pitting and crevice corrosion typically increases with increasing contents of chromium, molybdenum and nitrogen.
Corrosion resistance of stainless steels is a function not only of composition, but also of heat treatment, surface condition, and fabrication procedures, all of which may change the thermodynamic activity of the surface and thus dramatically affect the corrosion resistance. It is not necessary to chemically treat stainless steels to achieve passivity. The passive film forms spontaneously in the presence of oxygen. Most frequently, when steels are treated to improve passivity (passivation treatment), surface contaminants are removed by pickling to allow the passive film to reform in air, which it does almost immediately. Most of the ferritic and martensitic stainless steels have limited corrosion resistance in marine environments, but some of the newly developed ferritic grade s (often called “superferritics”) have excellent marine corrosion resistance and are widely used in applications such as tubes for power plant condensers.
Stainless steel can corrode in service if there is contamination of the surface. Both pickling and passivation are chemical treatments applied to the surface of stainless steel to remove contaminants and assist the formation of a continuous chromium-oxide, passive film. Pickling and passivation are both acid treatments and neither will remove grease or oil. If the fabrication is dirty, it may be necessary to use a detergent or alkaline clean before pickling or passivation. (reference)
Stainless Steel Weld Decay
This type of intergranular corrosion can occur in the heat-affected zone of welded components and also in cast components of stainless steel due to precipitation, during cooling, of chromium carbides at the grain boundaries (and hence loss of chromium in the immediately-adjacent zone). The local loss in corrosion resistance arises because the chromium is crucial in promoting the formation of a Cr-rich passive film on the surface of stainless steels. The susceptibility to weld decay can be counteracted by carrying out a suitable post-weld heat treatment to restore a uniform composition at the grain boundaries but this is clearly often not a practicable proposition. Consequently the usual strategy in combating weld decay is by the choice of stainless steel with either of the two following features: a.
specification of a stainless steel containing a small amount of either titanium or niobium; which have a higher affinity than does chromium for carbon: hence carbides of these elements tend to form instead of chromium carbides, thus avoiding the Cr-depletion problem: such steels are usually termed “stabilised stainless steels” b.
specification of a stainless steel with low carbon content (< 0.03%); this will clearly decrease the likelihood of carbide formation in the steel. Such low-carbon grades of stainless steel are often designated by a “L” in their code; for instance the “316” grade of steel (18%Cr/10Ni/2.5Mo) is designated as “316L” when its carbon content has been limited in this way.
Austenitic stainless steels suffer from stress corrosion cracking in hot solutions containing chloride. A high chloride concentration is required, although relatively small amounts of chloride are sufficient at heated surfaces, where chloride concentration can occur, or where chloride is concentrated by pitting or crevice corrosion, and problems can be experienced in tap water.
The temperature usually needs to be above 70°C, although SCC can occur at lower temperatures in some situations, notably more acid solutions. The cracking continues at low stresses and commonly occurs as a result of residual stresses from welding or fabrication. The cracking is normally transgranular, although it may switch to an intergranular path as a result of sensitization of the steel.
Rouging is a thin film, usually reddish-brown or golden in color, of iron oxide or hydroxide, typically on stainless steels. The contrast between this film and shiny metal accentuates this aesthetics problem. The rouge film typically wipes off easily with a light cloth (Figure 1), but it reforms while the process fluid is in contact with the stainless steel. This problem is most chronic in the pharmaceutical industry on the interior surfaces of high purity water (i.e., water for injection, WFI) distillation units, storage tanks, distribution systems (piping, valves, pump housings, fittings, etc.) and process vessels.