Stainless steels corrosion resistance, general

Corrosion. Aqueous solutions of citric acid are mildly corrosive toward carbon steels. At elevated temperatures, 304 stainless steel is corroded by citric acid, but 316 stainless steel is resistant to corrosion. Many aluminum, copper, and nickel alloys are mildly corroded by citric acid. In general, glass and plastics such as fiber glass reinforced polyester, polyethylene, polypropylene, poly(vinyl chloride), and cross-linked poly(vinyl chloride) are not corroded by citric acid. 

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... 

The corrosion resistance of nickel alloys has been extensively explored in seawater and saltwater (brackish water). Although stainless steel 316 is known to resist pitting in seawater, stainless steels are, in general, susceptible to pitting in the tidal zones of seawater. The nickel alloys, more expensive than steels, have been extensively used in seawater service. Inconel alloy 625 offers an excellent resistance to corrosion in seawater. It also offers an excellent resistance to SCC. Nickel alloys are best used for pump shafts, bodies and impellers while other materials, like 90-10 Cu-Ni and austenitic steels are used for other parts, such as heat exchangers and valves. Table 9.47 shows the classification of selected nickel alloys in seawater service. 

Because of the low viscosities of cryogenic Hquids, rolling element bearings seem better suited than hydrodynamic bearings for turbo pumps. AISI 440C stainless balls and rings generally are preferred for their corrosion resistance over the more commonly used AISI 52100 steel. 

The higher boiling phenols, present in considerable amounts in CVR and low temperature tars, are corrosive to mild steel, especially above 300°C. Cast iron, chrome steel, and stainless steel are more resistant. Furnace tubes, the insides of fractionating columns, and the rotors of pumps handling hot pitch and base tar are generally constmcted of these metals. Nevertheless, to ensure satisfactory furnace tube life, particularly in plants processing CVR or low temperature tars, the tube temperature should be kept to a minimum. 

The stainless steels contain appreciable amounts of Cr, Ni, or both. The straight chrome steels, types 410, 416, and 430, contain about 12, 13, and 16 wt % Cr respectively. The chrome—nickel steels include type 301 (18 wt % Cr and 9 wt % Ni), type 304 (19 wt % Cr and 10 wt % Ni), and type 316 (19 wt % Cr and 12 wt % Ni). Additionally, type 316 contains 2—3 wt % Mo which gready improves resistance to crevice corrosion in seawater as well as general corrosion resistance. AH of the stainless steels offer exceptional improvement in atmospheric conditions. The corrosion resistance results from the formation of a passive film and, for this reason, these materials are susceptible to pitting corrosion and to crevice corrosion. For example, type 304 stainless has very good resistance to moving seawater but does pit in stagnant seawater. 

Nickel and Nickel Alloys A wide range of ferrous and nonfer-rous nickel and nickel-bearing alloys are available. They are usually selected because of their improved resistance to chemical attack or their superior resistance to the effects of high temperature. In general terms their cost and corrosion resistance are somewhat a func tion of their nickel content. The 300 Series stainless steels are the most generally used. Some other frequently used alloys are hsted in Table 10-35 together with their nominal compositions. For metallurgical and corrosion resistance data, see Sec. 28. 

Corrosion resistance is inferior to that of austenitic stainless steels, and martensitic steels are generally used in mildly corrosive environments (atmospheric, fresh water, and organic exposures). 

The internals of the reservoir should be coated unless the reservoir is constructed of 300 series stainless steel. API 614 mandates the use of 304L, 321, or 347 stainless steel processed to ASTM A 240. For the critical equipment units, the stainless steel reservoir is a good idea. A decision must be made by the user relative to the general purpose units. A good coating can keep the internals clean and free of corrosion if applied properly. When the idea of stainless steel reservoirs was introduced, it met with immediate resistance, but as the alternatives were considered, it began to gain acceptance. 

Although transformers suitable for other industrial installations are generally suitable for producing applications, certain options may be desirable— primarily due to environmental considerations. At locations subject to harsh environmental conditions, and particularly at locations subject to washdown with high-pressure hoses, non-ventilated enclosures are desirable, if not necessary. Likewise, at locations subjected to salt water and salt-laden air, it often is desirable to specify copper windings and lead wires. Most manufacturers provide standard units with aluminum windings and lead wires. Even if aluminum coils are used, it is almost always desirable to require stranded copper lead wires. This will lessen corrosion and loose terminal problems when transformers arc interconnected to the facility electrical system with copper conductors. If the transformers are to be installed outdoors in corrosive environments, cases should be of corrosion-resistant material (e.g., stainless steel) or be provided with an exterior coating suitable for the location. 

In safety applications, the corrosion resistance of the duct materials deserv es special consideration. Since material costs generally increase along with corrosion resistance, the selection of material must be determined by the desired life span in the anticipated environment this environment is a function of the characteristics of the chemical being processed and the operating conditions of the reactor. For maximum resistance to moisture or corrosive gases, stainless steel and copper are used where their cost can be justified. Aluminum sheet is used where lighter veight and superior resistance to moisture are needed. 

Reactors should not dissolve in the reaction medium. Judging by spectro-graphic analysis of spent catalysts, some attack of the reactor is more common than is generally supposed. It may be a cause of catalyst failure. Reactors are commonly made of type 316 stainless steel, but other alloys may provide better resistance to spedhc corrosive agents. 

The scope of the term stainless steel has not been precisely defined, but for general purposes it may be considered to include alloys whose main constituent is iron but which also contain not less than 10% Cr. As with low-alloy steels, a distinction between low or medium carbon grades and high carbon grades must also be drawn, the latter being more in the nature of alloy cast irons. These are used mainly for oxidation resistance at high temperatures and for applications where abrasion resistance allied to a certain amount of corrosion resistance is required, and will not be considered in this section. 

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