Localized nickel-based alloys

An especially insidious type of corrosion is localized corrosion (1—3,5) which occurs at distinct sites on the surface of a metal while the remainder of the metal is either not attacked or attacked much more slowly. Localized corrosion is usually seen on metals that are passivated, ie, protected from corrosion by oxide films, and occurs as a result of the breakdown of the oxide film. Generally the oxide film breakdown requires the presence of an aggressive anion, the most common of which is chloride. Localized corrosion can cause considerable damage to a metal stmcture without the metal exhibiting any appreciable loss in weight. Localized corrosion occurs on a number of technologically important materials such as stainless steels, nickel-base alloys, aluminum, titanium, and copper (see Aluminumand ALUMINUM ALLOYS Nickel AND nickel alloys Steel and Titaniumand titanium alloys). 

M. Henthome, Intergranular Corrosion in Iron and Nickel-Base Alloys, Localized Corrosion Cause of Metal Failure, STP 516, ASTM, 1972, p 6-119... 

The insulation supplied by C-E is of the stainless steel reflective type, which minimizes insulation contamination in the event of a chemical solution spillage. In local areas around stainless steel and the nickel based alloy nozzles in the reactor vessel head, small sections of non-metallic insulation are used. However, the quantity of leachable halogens will be limited in accordance with Regulatory Guide 1.36. 

Kolts, J. and Sridhar, N., Temperature Effects in Localized Corrosion, Corrosion of Nickel-Base Alloys, R. C. Scarberry, Ed., ASM International, Metals Park, OH, 1985, pp. 191-198. 

Nickel-copper and nickel-chromium-molybdenum alloys are the nickel-base alloys that are t5fpically used in seawater. The nickel-copper alloys have good corrosion resistance in high velocity seawater, but do exhibit localized corrosion in quiescent seawater [79]. Alloy 625, a nickel-chromium-molybdenum alloy, is susceptible to crevice corrosion in both quiescent and flow conditions [97-700]. Other nickel-chromium-molybdenum alloys, such as Alloys C-276, C-22, 59 and 686 have increased seawater crevice corrosion resistance as compared to Alloy 625 [97,98],... 

Molybdenum, and sometimes tungsten, are also added to Ni-Cr-Fe allo3rs for improved localized corrosion (pitting and crevice corrosion) resistance. An example is alloy C-276 (UNS N10276), which contains about 57Ni-15.5Cr-15.5Mo-4W-5Fe and is one of the most pit-resistant nickel-base alloys available. 

Factors affecting localized corrosion of nickel-base alloys are chloride concentration, pH, temperature, crevice geometry (depth and tightness), and crevice former material (non-metallic or similar or dissimilar metal, metal surface condition, area ratio of exposed to shielded metal). A wide range of results can be obtained as these factors are varied. 

Nickel-base alloys respond well to most electrochemical test techniques and show active-passive behavior in many environments. Due to their rapid repassivation, however, the results obtained with potentiod3mamic techniques can sometimes be affected by scan rate and immersion time prior to starting the test [5,6], Electrochemical techniques are useful for investigating localized corrosion resistance, ASTM G 61, Test Method for Conducting Cyclic Potentio-dynamic Polarization Measurements for Localized Corrosion Susceptibility of Iron-, Nickel-, or Cobalt-Based Alloys, and general corrosion resistance, ASTM G 59, Practice for Conducting Potentiodynamic Polarization Resistance Measurements of nickel alloys. Electrochemical impedance measurement techniques have not been extensively applied to nickel alloys. 

New nickel-base alloys such as alloy G-30 (N06030), alloy 59 (N06059), and alloy 686 (N06686) have been developed with higher levels of chromium or molybdenum, or both, for increased resistance to localized or general corrosion in severe environments. These materials are being included in test procedures such as ASTM G 28. 

Among the nickel materials, the nickel-based alloys alloyed with chromium and molybdenum are to a great extent resistant to local corrosion in seawater, even at higher temperatures. They are used, even though the higher-alloyed stainless steels do not meet the requirements. 

Resistance to localized corrosion of stainless steels and nickel-base alloys in hot seawater - experience from the German North Sea... 

The corrosion behavior of non-ferrous alloys such as those based on nickel, cobalt, copper, zirconium, and titanium has been reviewed in detail in this chapter. Besides exotic materials such as tantalum and platinum, nickel-based alloys are the most resistant to corrosion by mineral acids, and they are especially resistant to localized corrosion in chloride-containing environments, which troubles stainless steels. Nickel-based alloys can broadly be divided into alloys, e.g. Ni-Mo (B-2, B-3) and Ni-Cu (alloy 400), that do not contain chromium, and are not, therefore, passivated under oxidizing conditions, and alloys, e.g. Ni-Cr-Mo (C-22, C-2000,59,686, etc.) and Ni-Cr-Fe (G-30, 825, etc.), that form a chromium oxide passive film under oxidizing conditions. Ni-Mo alloys such as B-3 have excellent corrosion resistance in hot reducing acids such as hydrochloric and sulfuric. Ni-Mo alloys cannot withstand oxidizing conditions such as nitric acid and hydrochloric acid contaminated with ferric ions. Ni-Cr-Mo alloys such as C-2000 alloy are multipurpose alloys that can be used both in reducing and oxidizing conditions. 

Cobalt-based alloys are usually less corrosion-resistant than nickel-based alloys, but are more resistant to wear. Co-Cr-Mo alloys such as Ultimet also have excellent resistance to chloride-induced localized attack. 

General corrosion is typically characterized by an oxidizing reaction which occurs uniformly over a material surface. This reaction causes a thinning of the surface, and corrosion proceeds until the surface fails by localized penetration or insufficient cross-sectional area to support a load. However, BWR internals are made from austenitic steel or nickel-base alloy with very low corrosion rates in the BWR environment. 

While general corrosion resistance is important, one of the major reasons that nickel-based alloys are specified for many applications is their excellent resistance to localized corrosion, such as pitting, crevice corrosion, and stress corrosion cracking. In many environments, austenitic stainless steels do not exhibit general attack but suffer from significant localized attack, resulting in excessive downtime and/or expensive repair and replacement. 

This technique has been especially useful to assess localized corrosion for passivating alloys such as S31600 stainless steel, nickel-based alloys containing chromium, and other alloys such as titanium and zirconium. Though the generation of the polarization scan is simple, its interpretation can be difficult [7]. 

The corrosion rates of steels and nickel-based alloys increase rapidly at temperatures above Tc [41] (Fig. 4.13). A local maximum in corrosion rate at temperatures around Tc has been predicted and observed (Section 4.4.3.1). 

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. 

Light-amber to yellow-red, oily fuming liquid with a suffocating, pungent, and nauseating odor. This material is hazardous through inhalation and ingestion, and produces local skin/eye impacts. It is noncorrosive to carbon steel and iron when dry. However, when wet it will attack steel, cast iron, aluminum, stainless steel, copper and copper alloys, and many nickel based materials. 

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... 

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