Stainless pitting corrosion

Pitting occurs witli many metals in halide containing solutions. Typical examples of metallic materials prone to pitting corrosion are Fe, stainless steels and Al. The process is autocatalytic, i.e., by initial dissolution, conditions are established which furtlier stimulate dissolution inside tire pit tire metal (Fe in tire example of figure C2.8.6 dissolves. 

The stainless steels contain appreciable amounts of Cr, Ni, or both. The straight chrome steels, types 410, 416, and 430, contain about 12, 13, and 16 wt % Cr respectively. The chrome—nickel steels include type 301 (18 wt % Cr and 9 wt % Ni), type 304 (19 wt % Cr and 10 wt % Ni), and type 316 (19 wt % Cr and 12 wt % Ni). Additionally, type 316 contains 2—3 wt % Mo which gready improves resistance to crevice corrosion in seawater as well as general corrosion resistance. AH of the stainless steels offer exceptional improvement in atmospheric conditions. The corrosion resistance results from the formation of a passive film and, for this reason, these materials are susceptible to pitting corrosion and to crevice corrosion. For example, type 304 stainless has very good resistance to moving seawater but does pit in stagnant seawater. 

Other passivating materials suffer pitting corrosion by chloride ions [62] in a way similar to stainless steels (e.g., Ti [63] and Cu [64]). The pitting potential for aluminum and its alloys lies between = -0.6 and -0.3 V, depending on the material and concentration of chloride ions [10,40-42]. 

Stainless steels in soil can only be attacked by pitting corrosion if the pitting potential is exceeded (see Fig. 2-16). Contact with nonalloyed steel affords considerable cathodic protection at f/jj < 0.2 V. Copper materials are also very resistant and only suffer corrosion in very acid or polluted soils. Details of the behavior of these materials can be found in Refs. 3 and 14. 

Figure 53.3 illustrates a pit in a stainless steel such as type 534 or 316 austenitic alloy. Pitting starts at heterogeneity in the steel surface, such as an outcropping sulfide inclusion, the shielded region beneath a deposit or even a discontinuity in the naturally present oxide film caused by a scratch or embedded particle of abrasive grit. This initiation phase of pitting corrosion may take seconds... 

Metals which owe their good corrosion resistance to the presence of thin, passive or protective surface films may be susceptible to pitting attack when the surface film breaks down locally and does not reform. Thus stainless steels, mild steels, aluminium alloys, and nickel and copper-base alloys (as well as many other less common alloys) may all be susceptible to pitting attack under certain environmental conditions, and pitting corrosion provides an excellent example of the way in which crystal defects of various kinds can affect the integrity of surface films and hence corrosion behaviour. 

Videm, K., Pitting Corrosion of Aluminium in Contact with Stainless Steel , Proc. Conf. on Corrosion Reactor Mater., Salzburg, Austria, 1, 391 (1962) C.A., 60, 1412g Lyon, D. H., Salva, S. J. and Shaw, B. C., Etch Pits in Germanium Detection and Effects , J. Electrochem. Soc., 110, 184c (1963)... 

Tomashov, N. D., Chernova, G. P. and Marcova, O. N., Effect of Supplementary Alloying Elements on Pitting Corrosion Susceptibility of 18Cr-14Ni Stainless Steel , Corrosion, 20, 166 (1964)... 

Schwenk, W., Theory of Stainless Steel Pitting , Corrosion, 20, 129 (1964)... 

Szklarska-Smialowska, Z., Electron Microprobe Study of the Effect of Sulphide Inclusions on the Nucleation of Corrosion Pits in Stainless Steels , Br. Corros. J., S, 159 (1970) Weinstein, M. and Speirs, K., Mechanisms of Chloride-activated Pitting Corrosion of Martensitic Stainless Steels , J. Electrochem. Soc., 117, 256 (1970)... 

Ijzermans, A. B., Pitting Corrosion and Intergranular Attack of Austenitic Cr-Ni Stainless Steels in NaSCN , Corros. Sci., 10, 607 (1970)... 

Rarey, C. R. and Aronson, A. H., Pitting Corrosion of Sensitised Ferritic Stainless Steel , Corrosion, 28, 255 (1972)... 

Kato, M., Inoue, T., Goto, K., Ito, G. and Shimizu, Y., Effect of Dissolved Oxidising Agents and Inhibitors on Pitting Corrosion of Aluminium in Water , Aluminium, 49, 289 (1973) Richardson, J. A. and Godwin, A. W., Localised Corrosion of Stainless Steel During Food Processing , Br. Corros. J., 8, 259 (1973)... 

Uhlig, H. H. and Gilman, 3. R., Inhibition of Pitting Corrosion of Stainless Steel 18/8 in Iron(III) Chloride Solutions by Nitrates , Z. Physik. Chem.,21/6, 127 (1964) C.A., 61,9231c Fisher, W. R., Pitting Corrosion, Especially of Titanium. 1 Corrosion Studies , Techn. Mill. 

Freiman, L. 1. and Kolotyrkin, Ya. M., Pitting Corrosion of Aluminium in Solutions of Sodium Perchlorate and Perchloric Acid , Zashch. Melal, 2, 488 (1966) C.A., 65, 19674d Novakovskii, V. M. and Sorokina, A. N., Comparative Electrochemistry of Stress Corrosion and Pitting of Stainless Steels in Chloride Solutions , Zashch. Melal, 2, 416 (1966) C.A., 65, 18152g... 

Rozenfd d, I. L. and Danilov, I. S., Electrochemistry of Pitting Corrosion, 1 Pit Formation During Spontaneous Dissolution of Stainless Steels , Zashch. Metal.,1, 134(1966) C.A.,65, 10114a... 

Takenori, N., Norio, S., Tatsuo, 1. and Okamoto, G., Electrochemical Test for Pitting Corrosion in Stainless Steels , Hakkaido Daigaku Kogakubu Kenkyu Hokoku, 44, 1 (1967) C.A., 70, 16534h... 

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

Herbsleb, G. and Schwenk, W., Flow Dependence of the Pitting Corrosion of Cr-Ni Steel in NaCl Solution. 2 Tests with Ultrasonics , Werkst. Korros., 24, 267 (1973) C.A., 79, 56638n El Din Shams, A. M., Bodran, M. M. and Khalil, S. E., Corrosion Behaviour of Manganese-containing Stainless Steel. 3 Their Susceptibility Towards Pitting Corrosion , Werkst. Korros., 24, 290 (1973) C.A., 79, 56642j... 

Imoi, H., Saito, Y., Kobayashi, M. and Fujiyama, S., Pitting-corrosion-resistant Chromium Stainless Steel , Japan Kokai 7300, 221 (1973) C.A., 79, 22569a Sato, E., Tamura, T. and Okabe, T., Aluminium Anode for Cathodic Protection. 7 Pitting and Corrosion Potentials for Gallium in Sodium Chloride Solutions , Kinzoku Hyomen Gijutsu, 24, 82 (1973) C.A., T9, 12792d... 

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