Nickel fogging

Inconel 600 remains bright indefinitely in indoor atmospheres, even in damp sulfurous atmospheres that may fog nickel 200 and Monel 400. However, because of cost Inconel 600 does not find many indoor applications. 

Sa.lt Spray Tests. One of the older accelerated corrosion tests is the salt spray test (40). Several modifications of this imperfect test have been proposed, some of which are even specified for particular appHcations. The neutral salt spray test persists, however, especially for coatings that are anodic to the substrate and for coatings that are dissolved or attacked by neutral salt fog. For cathodic coatings, such as nickel on steel, the test becomes a porosity test, because nickel is not attacked by neutral salt fog. Production specifications that call for 1000 hours salt spray resistance are not practical for quahty acceptance tests. In these cases, the neutral salt spray does not qualify as an accelerated test, and faster results from different test methods should be sought. 

The CASS Test. In the copper-accelerated acetic acid salt spray (CASS) test (42), the positioning of the test surface is restricted to 15 2°, and the salt fog corrosivity is increased by increasing temperature and acidity, pH about 3.2, along with the addition of cupric chloride dihydrate. The CASS test is used extensively by the U.S. automobile industry for decorative nickel—chromium deposits, but is not common for other deposits or industries. Exposure cycle requirements are usually 22 hours, rarely more than 44 hours. Another corrosion test, now decreasing in use, for decorative nickel—chromium finishes is the Corrodkote test (43). This test utilizes a specific corrosive paste combined with a warm humidity cabinet test. Test cycles are usually 20 hours. 

Soluble corrosion products may increase corrosion rates in two ways. Firstly, they may increase the conductivity of the electrolyte solution and thereby decrease internal resistance of the corrosion cells. Secondly, they may act hygroscopically to form solutions at humidities at and above that in equilibrium with the saturated solution (Table 2.7). The fogging of nickel in SO2-containing atmospheres, due to the formation of hygroscopic nickel sulphate, exemplifies this type of behaviour. However, whether the corrosion products are soluble or insoluble, protective or non-protective, the... 

Ni-30 Cu Alloy 400 also undergo fogging, but alloys containing 15% Cr or more do not exhibit this phenomenon. Fogging is prevented by a very thin film of chromium deposited on the surface—a fact which forms the basis for the bright appearance of decorative chromium-nickel plate (see Sections 13.7 and 13.8). 

Fogging reduction of the lustre of a metal by a film or particulate layer of corrosion product, e.g. the dulling of bright nickel surfaces. 

Limiting factors in the autoxidation pathways are the total concentration of the active metal catalyst and its equilibrium speciation as a function of pH. Los Angeles fog water contains high concentrations of iron, manganese, copper, nickel and lead, as shown in Figure Id. Of these metals, Fe, Mn and Cu are expected to be the most effective catalysts for the reaction of S(IV) with molecular oxygen (23, - ). Observed concentrations of Fe and Mn in fog of 400 and 15 respectively, were not unusual... 

General Corrosion, or Uniform Attack. This type of corrosion includes the commonly recognized rusting of iron or tarnishing of silver. Fogging of nickel and high-temperature oxidation of metals are also examples of this type. 

The atmosphere. In industrial atmospheres, a nonprotective film forms, composed of basic nickel sulfate (fogging). Fogging is minimized by a thin chromium electroplate over nickel. There is good resistance to oxidation in air at elevated temperatures. 

Nickel is sensitive to attack by industrial atmospheres and forms a film of basic nickel sulfate that causes the surface to fog or lose its brightness. To overcome this fogging, a thin coating of chromium (0.01-0.03 mil/0.003-0.0007 mm) is electrodeposited over the nickel. This finish is applied to all materials for which continued brightness is desired. 

A metal resists corrosion by forming a passive film on the surface. This film is naturally formed when the metal is exposed to the air for a period of time. It can also be formed more quickly by chemical treatment. For example, nitric acid, if applied to austenitic stainless steel, will form this protective film. Such a film is actually a form of corrosion, but once formed it prevents further degradation of the metal, provided that the film remains intact. It does not provide an overall resistance to corrosion because it may be subject to chemical attack. The immunity of the film to attack is a fimction of the film composition, temperature, and the aggressiveness of the chemical. Examples of such films are the patina formed on copper, the rusting of iron, the tarnishing of silver, the fogging of nickel, and the high-temperature oxidation of metals. 

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