General Corrosion Resistance

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. 

Generally, corrosion resistance is similar to gray iron. But duc tile iron can be used at higher temperatures—up to 590°C (I,I00°F) and sometimes even higher. 

Tables 2.3 through 2.5 give general corrosion-resistance ratings of different materials. Table 2.3 lists various metals and Table 2.4 gives ratings for various nonmetals. Table 2.5 gives typical corrosion rates of steel and zinc panels exposed to the atmosphere in various locations about the U.S. Figure 2.1 also illustrates relative corrosion rates of steel and zinc in major areas of the world.

These alloys offer several useful advantages. First, their general corrosion resistance is typically slightly above that of 316 L in most media. In addition, because the nickel content is held low, they offer very good resistance to chloride SCC. In combination with good corrosion resistance, duplex stainless alloys offer higher strengths than those typically found with austenitic steels. Table 4 compares some typical mechanical properties for common stainless and nickel alloys. 

The main approach to improving the pitting and crevice corrosion resistance of the basic 35% nickel, 19% chromium, and 2% molybdenum alloy was to increase the molybdenum content. Among the first of the newer alloys introduced was 904 L (UNS N08904), which boosted the molybdenum content to 4% and reduced the nickel content to 25%. The reduction in nickel content was beneficial as a costsaving factor, with minimal loss of general corrosion resistance and sufficient resistance to chloride SCC. 

In general, corrosion resistance is determined by the stability of the various phases present and the surface area available to attack. 

In oxidizing media and acids, titanium alloys are, in general, corrosion resistant, for example, in chromic, nitric, perchloric, and hypochlorus adds and their salts. 

Carbon steel shelters need to be painted to protect them from corrosion. Stainless steel shelters do not require to be painted, but are more expensive than carbon steel shelters. Aluminum has favorable weight-to-strength ratio. On roof tops and on other mounted stmctures, the dead weight of the shelter can be important for structural purposes. Aluminum is generally corrosion-resistant in nonmarine environments. Aluminum is costlier than carbon steel and requires no painting. 

Humidity resistance Grease and oil resistance General chemical resistance General corrosion resistance (industrial atmospheres) Exterior durability (pigment)... 

The thickness of the liner is a factor affecting permeation. For general corrosion resistance, thicknesses of 0.010-0.020 in. are usually satisfactory, depending on the combination of lining material and the specific corrodent. When mechanical factors such as thinning to cold flow, mechanical abuse, and permeation rates are a consideration, thicker linings may be required. 

Nitride layers are produced after treatment in a salt bath or in a gas atmosphere by inward diffusion, usually below 600°C. Unlike transformation hardening, which is effected by lattice distortion as a result of embedded carbon atoms, nitriding and boriding provide the surface with an exceptional increase in hardness by the formation of an intermetallic bonding layer. These bonding layers also increase the general corrosion resistance, but they are so thin that they can break by mechanical point loading. 

SS Contains 18% chromium and 8% nickel. It has good general corrosion resistance in moderately oxidising environments. 

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