Chromium steel, erosion-corrosion

Because alterations to equipment design can be cumbersome and expensive, a more economical approach may be to change the metallurgy of affected components. Metals used in typical cooling water environments vary in their resistance to erosion-corrosion. Listed in approximate order of increasing resistance to erosion-corrosion, these are copper, brass, aluminum brass, cupronickel, steel, low-chromium steel, stainless steel, and titanium. 

The resistance of a metal to erosion-corrosion is based principally on the tenacity of the coating of corrosion products it forms in the environment to which it is exposed. Zinc (brasses), aluminum (aluminum brass), and nickel (cupronickel) alloyed with copper increase the coating s tenacity. An addition of V2 to 1)4% iron to cupronickel can greatly increase its erosion-corrosion resistance for the same reason. Similarly, chromium added to iron-base alloys and molybdenum added to austenitic stainless steels will increase resistance to erosion-corrosion. 

Problems with steam can occur in let-down valves as a result of erosion-corrosion. To prevent attack, hard facing (e.g., stellite) is commonly used when the pressure drop exceeds 150 to 200 psi (1,035 to 1,380 kPa). This limit can be raised to 500 psi (3,450 kPa) for clean, dry steam. Corrosion-erosion also occurs in wet steam. Carbon steel is unsatisfactory in wet steam when pvx, the product of the pressure (psia), velocity (ft/s), and wetness (% water) exceeds 1 x 105. Resistance to wet steam is enhanced by increasing both the metal hardness and chromium content. 

Particular attention should be given to compatibility of the materials used with regard to the water chemistry in order to prevent corrosion phenomena. For all equipment exposed to damp steam or to fluids which can cause severe erosion, corrosion and erosion resistant materials should be used. Low alloy steel containing chromium (Cr >0.5%) may be used. 

As a result the area just downstream from a widening of the pipe diameter will corrode preferentially. Figure 10.33 illustrates this phenomenon. It shows the rate of erosion corrosion of a 15% chromium steel sample in contact with water that contains sand. The rate of erosion corrosion is particularly high in the narrow part of the tube, as well as at a short distance downstream from the expansion. The material loss exhibits a maximum at a relative distance where the fluctuation of the velocity vector perpendicular to the wall is greatest [20]. Under the conditions of the experiments the... 

Stainless steels of all grades, in general, are resistant to erosion corrosion. The addition of nickel, chromium, and molybdenum further improves their performance. Stainless steels and chromium steels are resistant as a result of their tenacious surface films. 

The erosion-corrosion resistance of uncoated and aluminized 12% chromium ferritic steels under fluidized-bed conditions at elevated temperature (SUNASPO)... 

SEM photographs of the surfaces of uncoated 12% chromium steel samples after the erosion-corrosion tests under various erosion-corrosion conditions (a) 7 m s 550°C, 30°, (b) 9 m s 550°C, 30°... 

Erosion-corrosion resistance of uncoated and pack-alumlnlzed ferritic 12% chromium steels was studied under fluidized-bed conditions in a fluidized-bed erosion-corrosion test rig. The erosion-corrosion experiments were conducted in the temperature range from 550°C to 700°C using silica sand particles as an erodent. The speed between the sample surfaces and the silica sand particles was varied from 7.0 m s to 9.5 m s and the impact angles of interest were 30° and 90°. The main results can be summarized as follows ... 

The erosion-corrosion behaviour of the aluminized 12% chromium steel changes in the temperature range from 600°C to 650°C. This is due to a shift in the erosion behaviour of the coating from brittle to ductile , and to a more rapid oxide scale build-up at temperatures above 600°C. 

If the amount of metal removal by erosion is significant the surface will probably be continually active. Metal loss will be the additive effect of erosion and active corrosion. Sometimes the erosion rate is higher than that of active corrosion. The material selection judgment can then disregard coirosion and proceed on the basis of erosion resistance provided the corrosion rates of aetive surfaces of the alloys considered are not much different. As an example of magnitudes, a good high-chromium iron may lose metal from erosion only a tenth as fast as do the usual stainless steels. 

In the polyacrylic synthetic fibre industry, carbonitrided molybdenum guides have been used in place of chromium plated steel because of their resistance to corrosion and erosion. Chemicals that attack molybdenum are listed in Table 5.9. 

The behavior of materials, particularly steel, in cavitating fluids results in an erosion mechanism, including mechanical erosion and electrochemical corrosion. The straightforward way to fight cavitation is to use hardened materials, chromium, chrome-nickel compounds, or elastomeric plastics. Other cures are to reduce the vapor pressure with additives, reduce the turbulence, change the liquid s temperature, or add air to act as a cushion for the collapsing bubbles. 

Investigators determined that a carbon steel section of 6-inch (150 mm) line was installed in an area in which specifications required corrosion-resistant 5-percent chromium—1/2-percent molybdenum (generally called 5-chrome) alloy piping. In a process plant like a fluid coker, the materials used are a mixture of carbon steel and other steel alloys. Apparently, the welder and the maintenance crew, who previously repaired this piping some time ago, were not aware of the piping specifications. Apparently, they did not realize that if a material like carbon steel was installed in an area requiring 5-chrome alloy piping, erosion or corrosion could cause failure. 

Chromium is an element with the symbol Cr and atomic number 24. This metal is highly valued because of its hardness and corrosion resistance. See Fig. 4 [20]. This metal is added to steel to form stainless steel. It is also electroplated onto other materials to protect them from corrosion. Chromium compounds are found in the enviromnent as a result of erosion of chromium-containing rocks, volcanic eruptions, and the dumping of chromium wastes from facihty production (in landfills). Chromium is mined as chromite (FeCr204) in places like South Africa, India, Russia, and Turkey [21]. 

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