Stainless atmospheric corrosion

Nickel and nickel alloys possess a high degree of resistance to corrosion when exposed to the atmosphere, much higher than carbon and low-alloy steels, although not as high as stainless steels. Corrosion by the atmosphere is, therefore, rarely if ever a factor limiting the life of nickel and nickel alloy structures when exposed to that environment. 

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. 

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. 

Atmospheric Atmospheric corrosion due to the combined effects of rain and the deposition of salt and other pollutants will affect most equipment. Corrosion occurs while the metal surface is wet, and is strongly influenced by the composition of deposits (such as sulfates from industrial atmospheres and chlorides from marine atmospheres). External corrosion of steel and stainless steel process equipment beneath thermal insulation and fireproofing is of particular concern. 

The shiny appearance, low-weight per volume, favorable mechanical properties such as material strength, ease of forming, and handling are some favorable features of aluminum and hence its use in the food industry. In addition, aluminum has a better corrosion resistance than carbon steel as it readily forms a protective film that prevents further atmospheric corrosion. Aluminum is also lighter than stainless steel and hence its use in beverage cans. 

In the case, that pipes cross other metallic components, even if they are made of the same material, the choice about which one must be located above the other can be influenced by considerations about the possible damage caused on the underlying material, not only by the percolation of atmospheric corrosion products but also by the accidental spillage of liquids for example, it is preferable that electrical cables and gas pipes pass over and not under the sewage. In case of fire, stainless-steel structures can be anbrittled from contact with molten zinc it is therefore appropriate that the... 

Stainless a-Fe203, Fe304 Atmospheric corrosion at high ... 

For the sake of consistency, a terminology has developed within the atmospheric corrosion testing community with respect to testing. The test facility itself is commonly referred to as a corrosion yard. Racks, commonly fabricated from pipe or wood, are installed to hold the fixtures for the individual test specimens. The latter are referred to as "frames," and can be fabricated from wood or metal (nickel-copper alloy, stainless steel, aluminum, etc.). These frames are usually designed to hold approximately 75 specimens, isolated from each other and the test frame by porcelain, plastic, or rubber insulators." Thus, a corrosion yard... 

Determination of resistance to intergranular corrosion of stainless steels—Part 2 Ferritic, austenitic and ferritic-austenitic (duplex) stainless steels—Corrosion test in media containing sulfuric acid Corrosion of metals and alloys— Determination of dezincification resistance of brass Copper alloys— Ammonia test for stress corrosion resistance Corrosion tests in artificial atmosphere—General requirements... 

Resistance to atmospheric corrosion was one of the earliest characteristics of the chromium-bearing alloys. When compared to iron and steel, their resistance prompted the name stainless. ... 

Although atmospheric corrosion of ferrous metals is related first to the amount of moisture in the air (relative humidity), atmospheric moisture alone has no influence on the corrosion of stainless steel. Of primary importance are the effects of such atmospheric contaminants as particulate chlorides and iron-based dust. Sulfur-based acids will promote corrosion while oxides of nitrogen improve the resistance. Contaminants such as hydrocarbons, ammonia, and oxides of carbon have no effect. 

The ferritic stainless steels contain 15-30% chromium with a low carbon content (0.1%). Resistance to atmospheric corrosion of the ferritic grades will depend on the chromium content as well as the condition of exposure. 

The duplex stainless steels are alloys whose microstructures are a mixture of austenite and ferrite. Type 32550 and related alloys containing molybdenum have an atmospheric corrosion resistance superior to type 316 in high-chloride environments. The duplex grade would be used when the greater strength of these alloys is required for application in a marine atmosphere. 

Stainless steel can best be protected from atmospheric corrosion by proper design techniques. It is essential that the design permit washing away of deposits by rain. If the stainless steel is partially sheltered deposits may be sufficiently hygroscopic and corrosive to lead to some attack that would not otherwise be encountered in the same location. 

Types 304 and 304L stainless steels are normally used to resist atmospheric corrosion. The importance of design was illustrated at a location where 304 stainless steel boldly exposed surfaces were virtually unattacked, whereas in the areas p2utially sheltered by the roof only type 316 and 317 stainless steels remained unattacked as a result of the molybdenum content. 

Aluminum has exceUent resistance to atmospheric sulfides and other pollutants. Its resistance to atmospheric corrosion is due to a tightly adherent oxide film, and destruction of this film by either mechanical or chemical means exposes a very reactive surface. If the protective oxide film is disturbed, the presence of salts, including chlorides, can cause rapid pitting of aluminum. Further, electrical coupling to iron, stainless steel, or copper will accelerate this deterioration. 

Chromium readily forms an oxide that is transparent and happens to be extremely resistant to further degradation. It is less noble than iron and, when alloyed with steel, tends to form its oxide first. Gradually increasing the chromium content in steel above the 2% level steadily improves mild atmospheric corrosion resistance up to approximately 12%, where corrosion is essentially stopped. For exposure to mild wet environments, the addition of approximately 11% chromium is sufficient to prevent rusting of steel, hence the term stainless. 

Ferritic stainless steels offer useful resistance to mild atmospheric corrosion and most freshwaters. They will corrode with exposure to seawater atmospheres. These alloys are also useful in high-temperature situations, with 446... 

Type 414 stainless steel is resistant to mild atmospheric corrosion, fresh water, and mild chemical exposures. Applications include high-strength... 

Clad transition metal systems provide an interface between two incompatible metals. They not only reduce galvanic corrosion where dissimilar metals are joined, but they also allow welding techniques to be used when direct joining is not possible. Qad metals provide an ideal solution to the materials problem of dual environments. For example, in the application of small battery cans and caps, copper-clad, stainless steel-clad nickel (Cu/SS/Ni) is used where the external nickel layer provides atmospheric corrosion resistance and low contact resistance. The copper layer on the inside provides the electrode contact surface as well as compatible cell chemistry. The stainless steel layer provides strength and resistance to perforation corrosion. 

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