Pitting corrosion resistance

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... 

Use of material with good pitting corrosion resistance is desirable in seawater and oilfleld brine (formation water) media. 

Kold, J., Anne, R. Moeller B, Influence of various surface conditions on pitting corrosion resistance of stainless steel tubes type EN 1.4404, presented at NACE corrosion, paper 06095, San Diego, 2006. 

Tsujikawa, S., Kudo, K., Ogawa, H., et al., "A New Test for Predicting Pitting Corrosion Resistance of CRA s in Sour Environments, Paper 65, NACE CORROSION/88 Conference, NACE International, St. Louis, MO, 1988. 

Varjonen et al. [25] studied the pitting corrosion resistance of Type 316 stainless steel in white waters containing thiosulfate using an Avesta Cell, where the pitting breakdown could be studied without interference from crevice corrosion. Salonen et al. [26] and Laitinen et al. [27] used the contact electric resistance (CER) method to study the effect of thiosulfate on the films formed on Type 304 stainless steel. In the CER method, the resistance of the films formed on metal surfaces can be studied as a function of potential. Stable passive films have a high electrical resistance, whereas unstable (or absent) films have a low electrical resistance. 

This allows the pitting corrosion resistance to be estimated from the composition of the steel. The differing efficiencies of the alloying elements is taken into account in the pitting resistance equivalent (PRE). 

The pitting corrosion resistance of the steels increases as the value of the pitting resistance equivalent increases. 

The benefit of nitrogen alloying appears to be more oblique - the delay in M23C6 precipitation and improved pitting corrosion resistance has been recognized to be beneficial against SCC since pits are likely to initiate SCC. 

Ce-Mo process. Treatment in the Ce-Mo process resulted in a very corrosion resistant snrface with a high value. Further illustration of the marked improvements in corrosion resistance of Al 6061 is shown in Fig. 2.8, where the impedance only show little decrease after the treated sample was exposed to 0.5 N NaCl for 30 days. The impedance spectra shown in Fig. 2.8 for Al 6061 are very similar to those commonly observed for stainless steels in NaCl solutions. Mansfeld and Perez also developed a process for the surface modification of AA6061-T6 by immersion in a mixture of molten salts such as a NaCl-SnClj-CeCl3 melt at 200 °C. Samples treated in molten salts for 2 hours were subsequently exposed to a NaCl solution for 30 days to evaluate resistance against pitting corrosion by EIS. The results showed that this coating technique enhanced the pitting corrosion resistance of alitmimtm alloys in contact with seawater. 

The conclusion is that the larger the sulfide solubility in the corrosive solution is, the lower the pitting resistance. The pitting corrosion resistance increases then in the same way that the solubility of sulfides Ca < Mn < Cr < Ti < Ce. Finally, the role of the sulfides in pit initiation could be simply to provide sulfur species more or less easily (following the sulfur stability), which deposit then on the passive film around the sulfide, resulting in a locally poorer pitting resistance. 

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