Nickel, atmospheric corrosion

Tin—Nickel. AHoy deposits having 65% fin have been commercially plated siace about 1951 (135). The 65% fin alloy exhibits good resistance to chemical attack, staining, and atmospheric corrosion, especially when plated copper or bron2e undercoats are used. This alloy has a low coefficient of friction. Deposits are solderable, hard (650—710 HV ), act as etch resists, and find use ia pfinted circuit boards, watch parts, and as a substitute for chromium ia some apphcafions. The rose-pink color of 65% fin is attractive. In marine exposure, tin—nickel is about equal to nickel—chromium deposits, but has been found to be superior ia some iadustfial exposure sites. Chromium topcoats iacrease the protection further. Tia-nickel deposits are bfitde and difficult to strip from steel. Temperature of deposits should be kept below 300°C. 

In the massive state none of these elements is particularly reactive and they are indeed very resistant to atmospheric corrosion at normal temperatures. However, nickel tarnishes when heated in air and is actually pyrophoric if very finely divided (finely divided Ni catalysts should therefore be handled with care). Palladium will also form a film of oxide if heated in air. 

Fig. 4.38 Atmospheric corrosion of nickel and nickel alloys during exposure tests at sites in the USA. 1, Nickel 200 2, Alloy 600 3, Alloy 800 4, Alloy 825 5, Alloy 400 (after van Rooyen...

Fig. 12.9 Corrosion resistance of tin-nickel electrodeposit impaired by pseudomorphic porosity originating on cold-rolled steel surface (left). Panel on right has had the shattered grain surface removed by chemical polishing (0-125 iim removed). Coating thickness 15 iim-, panels exposed 6 months to marine atmospheric corrosion (Hayling Island)...

While the chemical resistance varies somewhat, stainless steel is fairly resistant to most acids and bases, is not amalgamated by mercury, and is generally resistant to oxidizing agents. While it can be used in fluorine handling, Monel and nickel are much better for this purpose. The resistance of stainless steel to atmospheric corrosion is an advantage in vacuum work because a corroded surface tends to outgas. 

Although the degree of atmospheric corrosion of copper and its alloys depends upon the corrosive agents present, the corrosion rate has been found to generally decrease with time. The copper and its alloys such as silicon bronze, tin bronze usually corrode at moderate rates, while brass, aluminum bronze, nickel silver, and copper-nickel corrode at a slower rate.51 The most commonly used copper alloys are Cl 1000, C22000, C38500 and C75200. 

Materials such as metals, alloys, steels and plastics form the theme of the fourth chapter. The behavior and use of cast irons, low alloy carbon steels and their application in atmospheric corrosion, fresh waters, seawater and soils are presented. This is followed by a discussion of stainless steels, martensitic steels and duplex steels and their behavior in various media. Aluminum and its alloys and their corrosion behavior in acids, fresh water, seawater, outdoor atmospheres and soils, copper and its alloys and their corrosion resistance in various media, nickel and its alloys and their corrosion behavior in various industrial environments, titanium and its alloys and their performance in various chemical environments, cobalt alloys and their applications, corrosion behavior of lead and its alloys, magnesium and its alloys together with their corrosion behavior, zinc and its alloys, along with their corrosion behavior, zirconium, its alloys and their corrosion behavior, tin and tin plate with their applications in atmospheric corrosion are discussed. The final part of the chapter concerns refractories and ceramics and polymeric materials and their application in various corrosive media. 

Each metal behaves in a unique way with respect to atmospheric corrosion properties, and the conclusions drawn from the nickel study cannot necessarily be drawn for other metals. However, if the same or similar corrosion products are formed on a given metal when exposed to a laboratory and a natural atmospheric environment, respectively, the results surest that the same corrosion processes are operating in both exposures. Table 4 displays examples of reported laboratory tests that have generated corrosion products similar to those seen in natural field exposures [13-18]. It appears that certain combinations of two or three corrodents at concentrations below 1 ppmv, together with a proper choice of relative humidity and airflow rate, can generate the corrosion products that are formed in natural field environments. 

In wet atmospheres, nickel initially forms NiO and (NiOH)2 [35,36]. Nickel sulfates are present as corrosion products on the surface in outdoor exposures [37].Jouenefatmospheric corrosion of nickel in industrial, urban, and rural atmospheres. Nickel corrodes through a pitting corrosion process. The highest corrosion rates were observed in industrial areas. The corrosion products were mainly sulfates, chlorides, and n ligible amounts of nitrates surrounded by carbonate species. The pitting corrosion process occurs in two steps on nickel surfaces exposed to an outdoor atmosphere, as shown in Fig. 10.9 [38]. 

S. Jouen, M. Jean, B. Hannoyer, Atmospheric corrosion of nickel in various outdoor environments, Corros. Sci. 46 (2004) 499-514. 

Copper and copper alloys are highly resistant to atmospheric corrosion because of surface films mainly composed of basic copper salts. The corrosion rate is below 2-3 pm/year [8.9]. Tin as well as nickel and nickel alloys also corrode at similar rates. Lead possesses excellent corrosion resistance in atmospheres due to surface-protecting films (insoluble sulphate, sulphide, carbonate and oxide). 

Nickel is quite resistant to marine atmospheres, but is sensitive to sulfuric acid of industrial atmospheres (Table 9.2), forming a surface tarnish composed of basic nickel sulfate. Corrosion in the industrial atmosphere of New York City is about 30 times higher than in the marine atmosphere of La Jolla, California, and about 20 times higher than in the rural atmosphere of State College, Pennsylvania (Table 9.2). 

---