Iron-base alloys pitting corrosion

K. OsozawaandN. Okato, Effect of Alloying Elements, Especially Nitrogen, on Initiation of Pitting in Stainless Steel, Passivity and Its Breakdown in Iron and Iron-Base Alloys, R.W. Staehle and H. Okada, Ed., National Association of Corrosion Engineers, 1976, p 135-139... 

Replacing some of the nickel with iron produces a family of alltws with intermediate corrosion resistance between stainless steels and the Ni-Cr-Mo alloys. Alloys such as Incoloy 825 and Hastelloy G-3 and G-30 are in this family. Incoloy 825 has 40 percent Ni, 21 percent Cr, 3 percent Mo, and 2.25 percent Cu. Hastelloy G-3 contains 44 percent Ni, 22 percent Cr, 6.5 percent Mo, and 0.05 percent C maximum. These alloys have extensive applications in sulfuric acid systems. Because of their increased nickel and molybdenum contents they are more tolerant of chloride-ion contamination than are standard stainless steels. The nickel content decreases the risk of stress-corrosion cracking molybdenum improves resistance to crevice corrosion and pitting. Many of the nickel-based alloys are proprietary and are coverecf by the following specifications ... 

A similar reaction occurs during pitting corrosion of iron and its alloys. Partial hydrolysis, leading to the formation of Al(OH) and Al(OH) may also occur, but all such reactions lead to the formation of acid, making the solution inside the pit much more aggressive than outside. Measurement of the pH inside a pit is not an easy matter, but estimates based on various calculations and on measurements in model pits lead to values as low as 1-2 for chromium-containing ferrous alloys and about 3.5 for aluminum-based alloys, depending on experimental conditions. 

Pitting corrosion is usually associated with active-passive-type alloys and occurs under conditions specific to each alloy and environment. This mode of localized attack is of major commercial significance since it can severely limit performance in circumstances where, otherwise, the corrosion rates are extremely low. Susceptible alloys include the stainless steels and related alloys, a wide series of alloys extending from iron-base to nickel-base, aluminum, and aluminum-base alloys, titanium alloys, and others of commercial importance but more limited in use. In all of these alloys, the polarization curves in most media show a rather sharp transition from active dissolution to a state of passivity characterized by low current density and, hence, low corrosion rate. As emphasized in Chapter 5, environments that maintain the corrosion potential in the passive potential range generally exhibit extremely low... 

The major alloying element contributing to resistance to pitting corrosion in iron- and nickel-base alloys is chromium. The effect of chromium in reducing both the critical current density and the passivating potential of iron in 1 N H2S04 is shown by the polarization curves of... 

ASTM G 61, Test Method for Conducting Cyclic Potentio-dynamic Polarization Measurements for Localized Corrosion Susceptibility of Iron-, Nickel-, or Cobalt-Based Alloys— This standard presents a procedure for performing cyclic poten-tiodynamic polarization testing to determine the relative susceptibility of iron-, nickel-, and cobalt-based alloys to localized corrosion (pitting or crevice corrosion). It illustrates and presents test apparatus, reagents, and materials, test procedures, and interpretation of results. 

Cyclic Potentiodynamic Polarization Measurements for Localized Corrosion Susceptibilily of Iron-, Nickel-, or Cobalt-Based Alloys, is quite effective [8,9,22]. A sample curve showing the effects of a corrosion inhibitor is in Fig. 1. An increase in the pitting (Ep), and breakdown (E),) potentials is indicative of a good anodic inhibitor. 

Frequent use has been made of ASTM G 48, Test Methods for Pitting and Crevice Corrosion Resistance of Stainless Steels and Related Alloys fy the Use of Ferric Chloride Solution. Specimens are immersed in ferric chloride or acidified ferric chloride and are evaluated by visual examination and mass loss. A related document. Guide for Crevice Corrosion Testing of Iron-Base and Nickel-Base Stainless Alloys in Seawater and Other Chloride-Containing Aqueous Environments is found in ASTM G 78. 

Although it is a matter of common knowledge that stainless steel is quite prone to corrosion in fuel cells, bare substrates of different alloys were tested in past material investigations. In 1998, Hornung and Kappelt (1998) selected different iron-based materials for Solid Polymer Fuel Cell bipolar plates by using the pitting resistance equivalent (PRE = %Cr + 3.3%Mo + 30%N) as corrosion resistance criterion. The authors exposed that some iron-based materials with PRE >25 (the material compositions are not given... 

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