May 4, 2004
Stainless steels are inherently resistant to surface attack in mildly corrosive environments. However, when corrosion does occur, it can result in the formation of pits on the surface or within crevices of the part. Why does this situation develop, and what can be done to prevent catastrophic failure?
Stainless steel's corrosion resistance is due to a thin, chromium-rich, transparent oxide film on the surface1. This protective film develops when more than 10.5 percent of chromium is present in the alloy and when the gas or liquid environment the stainless steel is exposed to provides oxygen to its surface.
Under these conditions, the surface is passive, or resistant to corrosion. Failure to develop and maintain this passive film renders the surface active, or possessing corrosion resistance similar to conventional steel's or cast iron's. It is essential that the entire surface be in a passive condition. If small regions of the surface are active, they will be readily attacked in a corrosive environment. This condition can be measured using electrochemical techniques as a potential of 0.78 volt between those active regions and the adjacent passive regions. This condition will accelerate the rate of metal dissolution into the surrounding electrolyte.
Passivation is the process of forming a protective oxide film on stainless steel. However, some confusion still exists about the definition of passivation and what really causes a passive film to form on the surface of stainless steel. On one hand, ASTM A380-99 notes that "Passivation is a process by which a stainless steel will spontaneously form a chemically inactive surface when exposed to air or other oxygen-containing environments. It was at one time considered that an oxidizing treatment was necessary to establish this passive film, but it is now accepted that this film will form spontaneously in an oxygen-containing environment providing that the surface has been thoroughly cleaned or descaled."
On the other hand, ASTM A380-99 also notes that "Passivation is removal of exogenous or free iron or iron compounds from the surface of a stainless steel by chemical dissolution, most typically by a treatment with an acid solution that will remove the surface contamination but will not significantly affect the stainless steel itself. Unless otherwise specified, it is this definition of passivation that is taken as the meaning of a specified requirement for passivation."
Thus, passivation may refer to the chemical removal of free iron or iron compounds from the surface, or it may refer to the spontaneous development of a chemically inactive surface (protective oxide film) on the stainless steel. While it is essential that surface contamination be removed completely, it is the latter interpretation of passivation that relates to establishing the corrosion resistance of stainless steels. Once the surface is cleaned and the bulk composition of the stainless steel is exposed to air or an oxygen-containing chemical environment, the passive film forms immediately.
Passivation typically is accomplished either through an appropriate bright annealing of the stainless steel or by subjecting the surface to an appropriate chemical treatment. In both procedures the surface is cleaned of contaminants and the metal surface is subsequently oxidized.
Bright Annealing. Bright annealing entails heating the stainless steel to a suitably high temperature (usually more than 1,900 degrees F, or 1,040 degrees C) in a reducing atmosphere such as dry hydrogen gas. Organic contaminants are volatilized and most metal oxides (including those of iron, nickel, and chromium) will be reduced, resulting in a clean, oxide-free surface. The stainless steel then is rapidly cooled (through the temperature range of 1,600 and 800 degrees F, or 870 and 425 degrees C) to inhibit carbide precipitation, and then at lower temperatures exposed to air, where the protective oxide film forms spontaneously.
Chemical Treatment. Typical chemical treatment involves exposing the stainless steel surface to an oxidizing acid solution in which the significant variables are time, temperature, and concentration. Many combinations of these variables can be used, but two of the most common are:
1. 20 percent nitric acid at 70 to 120 degrees F (20 to 50 degrees C) for 20 to 120 minutes. Acid concentrations up to 50 percent can be used, and the solution and residual effluent must be monitored closely. While very effective as a passivator, this solution may have environmental ramifications.
2. 4 to 10 percent citric acid plus 0.5 to 2.0 percent EDTA (ethylene-diamine-tetraacetic acid) at 170 degrees F (77 degrees C) for one to 10 hours. EDTA is a chelating agent that keeps iron in solution over a wide pH range. This solution has high reactivity with free iron, is less sensitive to exposure time, is far less corrosive to other materials, is less costly, and is considered environmentally friendly when used properly.
Passivation results in the formation of an oxide film having a higher chromium-to-iron ratio than the underlying stainless steel because of the preferential oxidation of chromium and the preferential dissolution of iron. Best performance is achieved with a Cr-Fe ratio of the surface oxide of more than 1.5.
ASTM A380-99 sets forth several techniques to determine the presence of free iron (a measure of adequate passivation) on the surface of stainless steel. The most commonly used of these is the copper sulfate test, in which a sulfuric acid-copper sulfate solution is swabbed on the surface for six minutes. The presence of any free iron (inadequate passivation) is indicated by the deposition of copper on the surface where free iron is present. This test may be readily conducted on stainless steel sheet, tube, pipe, and fittings, as well as on welds and heat-affected zones (HAZ).
It should be stressed that the protective oxide film formed during effective passivation of the stainless steel is transparent and not observable to the naked eye. Stainless steel owes its corrosion resistance to its ready oxidation to form this protective film; however, stainless steel's exposure to an oxidizing environment at higher temperatures (or to a more highly oxidizing environment at a given temperature) will result in the formation of an oxide (heat tint) of increasing thickness, ranging in color from a light straw to a dark black. The thicker this heat tint oxide is, the greater the probability that corrosion will occur beneath the oxide film.
AWS's D18.2-99 Guide to Weld Discoloration Levels on Inside of Austenitic Stainless Steel Tubes standard depicts tints developed on welds and HAZs caused by variations in welding parameters. It is likely that the darker, thicker oxide tints are the result of the formation of an oxide with higher iron and magnesium content; that is, reduced chromium content.
This interior view of a weld made on 304L exhibits the formation of heat tint on the weld and HAZ caused by the presence of oxygen during orbital welding.
Preservation of the passive film requires the presence of oxygen in the environment to which the stainless steel is exposed. In many instances of passivation failure, or corrosion attack, the integrity of the passive oxide film is compromised by the concentration of halide ions, for example, chlorine, which chemically attacks the oxide film. Extreme chloride concentrations may develop because of evaporation in systems that are not properly drained, which negates the corrosion protection expected of stainless steels.
An example of reduced corrosion resistance is shown in Figure 1. In the interior of an orbitally welded section of 304L, the inner surface of the weld bead, and the adjacent HAZ, are covered with a varicolored oxide film, or heat tint. The color of this film is a function of the film thickness that developed at various distances from the weld. When the section was placed in service, a corrosive environment preferentially attacked the stainless steel surface under the heat tint.
After removal of the heat tint oxide, evidence of in-service pitting in the HAZ is apparent.
Evidence of in-service pitting in the heat tint in the HAZ may be seen in Figure 2. This heat tint must be removed before the part is placed in service for optimum performance. Light oxides can be removed with bright annealing when possible; light tints and iron contamination may be cleaned with citric acid solutions; darker tints may require cleaning with various pickling pastes; while heavier, darker oxide films will require pickling solutions. Fabricators, designers, and users of stainless steel components must understand the factors involved in successful passivation, identify the conditions under which that protective oxide film may be compromised, and use techniques that expose inadequate passivation.
Carl R. Loper Jr., Ph.D., P.E., is a retired professor of materials science and engineering at the University of Wisconsin-Madison and adjunct professor in the Materials Department at the University of Wisconsin-Milwaukee. He can be reached at 608-238-2401, fax 608-238-2459, firstname.lastname@example.org.
The author would like to thank Monty B. Kuxhaus of MK Services Inc. and David O'Donnell and Carl Kettermann of Rath Manufacturing Company Inc. for their assistance with this article.
1. This film is about 10 atoms, or 35 angstroms, thick (0.00035 microns, or 1.4x10-7 inches).
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