Austenitic stainless steels pitting corrosion

Figure 2.20 Austenitic stainless steel plate from plate-and-frame heat exchanger. The orange oxide was formed from corrosion product originating at the regularly spaced pits. Pits are present near points of contact between adjacent plates. Corrugations run at right angles on adjacent plates.

The shape of a vessel determines how well it drains (Figure 53.7). If the outlet is not at the very lowest point process liquid may be left inside. This will concentrate by evaporation unless cleaned out, and it will probably become more corrosive. This also applies to horizontal pipe runs and steam or cooling coils attached to vessels. Steam heating coils that do not drain adequately collect condensate. This is very often contaminated by chloride ions, which are soon concentrated to high enough levels (10-100 ppm) to pose serious pitting and stress corrosion cracking risks for 300-series austenitic stainless steel vessels and steam coils. 

Stainless steels are particularly prone to crevice corrosion, and even the Fe-18Cr-8Ni-3Mo type of austenitic stainless steel, which is highly resistant to pitting when the surface is free from crevices, is susceptible although initiation of attack may take 1-2 years... 

Suzuki, T., Yambe, M. and Kitomura, Y., Composition of an Anolyte Within Pit Anode of Austenitic Stainless Steel in Chloride Solution , Corrosion, 29, 18 (1973)... 

Brigham, R. J., Pitting of Molybdenum Bearing Austenitic Stainless Steel , Corrosion, 28, 177... 

Grassiani, M., Pit Corrosion of Austenitic Stainless Steels in Artificial Sea-water , Trib. CEBEDEAU, 25, 515 (1972) C.A., 79, 8531a... 

With small modifications, ASTM standard G48 can be used to determine a CPT. The test is used as a ranking parameter for the resistance to pitting of high-alloyed austenitic stainless steels. In this method, material coupons are typically exposed for 24 or 72 h to a 6% FeCl3 (=1.11 mole/liter) solution at fixed temperatures (typically with 2.5°C intervals). The CPT is defined as the lowest temperature at which the specimen is attacked by pitting corrosion. 

High chloride waters will increase the risk of stress corrosion cracking (SCC) in austenitic stainless steels (e.g., 304/304L, 316/316L) and will tend to both increase the general rate of corrosion and attack in localized areas, often causing pitting-type corrosion. 

Steel phases have an influence on the rate of corrosion. Ferrite has a weak resistance to pitting. The presence of martensite can increase the hydrogen fragilization of steel. Intermetallic phases as Fe2Mo in high Ni content alloys can influence the corrosion resistance. The precipitate CuA12 in aluminum alloys the series 2000 is more noble than the matrix, with corrosion around the precipitate. The majority of case histories reported in the literature have involved austenitic stainless steels, aluminum alloys, and to a lesser degree, some ferritic stainless steels and nickel-based alloys.31... 

The Tafel constant was b = 0.20 V decade-1 for iron electrodes [55] and b = 0.20 V decade-1 for austenitic stainless steels [54] in acid solution. It is noticed that these Tafel constants are greater than those (0.03-0.1 V) usually observed with general dissolution of metals in acid solution. The other mode of localized corrosion is the active mode of corrosion that prevails in the potential range less positive (more cathodic) than the passivation potential, EP, in which potential range the localized corrosion is mainly controlled by the acidity of the occluded pit solution. In the potential range of active metal dissolution, the anodic dissolution current density is also an exponential function of the electrode potential, except for diffusion-controlled dissolution. 

Fig. 7.48 Response of five austenitic stainless steels to pitting and crevice corrosion. Alloys exposed 1 month at room temperature in indicated concentrations of FeCI3 solutions. (Numbers represent weight percent.)...

T. Suzuki, M. Yamabe, and Y. Kitamura, Composition of Anolyte within Pit Anode of Austenitic Stainless Steels in Chloride Solution, Corrosion, Vol 29, 1973, p 18-23... 

R.J. Brigham and E.W. Tozer, Effect of Alloying Additions on the Pitting Resistance of 18% Chromium Austenitic Stainless Steel, Corrosion, Vol 30, 1974, p 161-166... 

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