Carbon steel corrosion pitting

Pitting (Sections 1.5 and 1.6) Pitting of carbon steel is seldom catastrophically rapid in service and can often be accommodated within the corrosion allowance for the equipment. It often takes place under scale or deposits so that regular descaling of equipment can be beneficial. 

Chlorides are responsible for the pitting corrosion of steel parts. Normal carbon steel can stand 1000 ppm of chlorides (=1000 g M 3), but stainless steel starts to corrode severely from 100 ppm on Attention for ladders, illumination sets etc. 

A particularly insidious failure mechanism that is commonly found in carbon-steel tubing is under-deposit corrosion. In many cases, corrosion products fomi a scab that can mask the presence of the pitting, making it difficult to quantitatively assess using conventional NDT methods. However, by combining proper cleaning procedures with laser-based inspection methods, the internal surface of the tubing can be accurately characterized and the presence of under-deposit corrosion can be confirmed and quantified. 

Stainless steels attacked by sulfate reducers show well-defined pits containing relatively little deposit and corrosion product. On freshly corroded surfaces, however, black metal sulfides are present within pits. Rust stains may surround pits or form streaks running in the direction of gravity or flow from attack sites. Carbon steel pits are usually capped with voluminous, brown friable rust mounds, sometimes containing black iron sulfide plugs fFig. 6.10). 

Which Alloy to Use. Unalloyed mild steel parts have been known to corrode at rates as high as 800 mils per year. The low-chrome steels, through 9-Cr, are sometimes much more resistant than mild steel. No corrosion has been reported, with both 2%-Cr and 5-Cr furnace tubes, whereas carbon steel tubes in the same service suffered severe coiTosion. The 12-Cr stainless steels are scarcely, if any, better than the low-chromes. But the 18-8 Cr-Ni steels, without molybdenum, are often quite resistant under conditions of low velocity although they are sometimes subject to severe pitting. 

Finally, a book has recently been published covering corrosion problems related to nuclear waste disposal" . It discusses a variety of subjects including corrosion behaviour and SCC of copper, carbon steels and high alloy steels under conditions related to nuclear waste disposal. Special attention is paid to pitting and problems associated with hydrogen gas generation from corrosion processes. 

This frontier s practical opportunities were first developed with submarines, which until the nuclear ones were limited to depths of only a few hundred feet. Many thousands of feet can now be navigated. The crushing pressures below the surface, which increase at a rate of about V2 psi per foot of depth, make corrosion a major threat to the operation and durability of many materials. For example, the life of uncoated magnesium bolts in contact with steel nuts is less than seventy-two hours, aluminum buoys will corrode and pit after only eleven months at just four hundred feet, and low-carbon steel corroded at a rate one-third greater than in surface waters. 

Corrosion has been encountered infrequently to date and has been a surface type, as opposed to pitting corrosion that can result in perforations. Entrapped air in the beverage or in the cans headspace increases the corrosive action of the product according to Koehler et at (21). As with beer and other canned foods, aluminum ends provide electrochemical protection when combined with tinplate or tin-free-steel can bodies. The level of iron pickup is reduced while the amount of aluminum dissolved in soft drinks increases without detrimental effect. Aluminum containers with vinyl epoxy and vinyl organosol coatings are compatible with carbonated soft drinks. 

A specific waterside problem that affects many economizers is normally one of oxygen corrosion. This affects the internal, carbon steel tube header, first-pass tubes, and primary tube bend areas because these areas first receive cold FW. This form of corrosion commonly results in red oxides, economizer pitting and tuberculation, and potentially premature tube failure. 

In the case of carbon steel, the accumulation of Fe2+ in the crevice attracts chloride ions (Cl-) and aggressive pitting corrosion may occur. 

Uniform and pitting-type corrosion of various materials (carbon steels, stainless steels, aluminum, etc.) could be characterized in terms of noise properties of the systems fluctuation amplitudes in the time domain and spectral power (frequency dependence of power) of the fluctuations. Under-film corrosion of metals having protective nonmetallic coatings could also be characterized. Thus, corrosion research was enriched by a new and sufficiently correct method of looking at various aspects of the action of corrosive media on metals. 

Customer s laboratory says there are severe pitting corrosion problems, especially under slimes. Corrosometer corrosion rates indicate 6 to 8 mpy on steel. Carbon steel coupons indicate 10 to 12 mpy. Copper is 0.4 mpy. 

The position of the water table determines the oxygen transport and hence the corrosion rate. The moisture content of soil greater than 20% is deemed to be corrosive (general corrosion of carbon steel) and the value of less than 20% was conducive to pitting corrosion.15 This observation is thought to be related to the diffusion rate of oxygen.16 The general effect of soil resistivity on the corrosivity may be denoted as ... 

The effect of the chromium content of the alloy on corrosion in boiling acids is shown in Table 4.7 along with the data for carbon steel and low-carbon and low-nitrogen 35% Cr alloys. The data show that the corrosion rates of 18 Cr-8 Ni (Type 304) is lower than Type 430 and 446 that is devoid of nickel. The nickel is the alloy probably reduces the rate of hydrogen evolution reaction. The molybdenum in Type 316 alloy was found to be useful in protection from pitting by chloride ions. 

Cooling water tubes are used in extensively industry. Corrosion of cooling water tubes or pipes is a common phenomenon. The heat exchanger is opened to examine the extent of corrosion. Heavy deposits are revealed inside the tubes (Figure 7.30). The outer surface of the carbon steel tubes are in good condition and free from pitting attack (Figure 7.31). 

Significant microbiologically induced corrosion due to the presence of bacteria in the water is evidenced by saucer-shaped pits, smooth sided pits, bright shiny copper to matte red clean areas. The black deposits, corrosion products from carbon steel, may cause underdeposit corrosion and may cause the failure. Treatment of the water with biocide may minimize microbiologically induced corrosion. 

The investigations on corrosion and corrosion inhibition were carried out under the conditions of the oxidation experiments and also at elevated temperatures up to 523 K. The metallic samples were not pre-treated. On the surface of carbon steel St 1203 as well as high-grade steel 1.4541 without addition of an inhibitor corrosion and pitting can clearly be seen after the... 

High solids (carbonates and bicarbonates) BFW will result in significant formation of carbon dioxide. The resulting carbon dioxide-laden condensate causes erosion-corrosion attack on carbon steel. Failure occurs by deep pitting, furrowing, or channeling. Corrosion inhibitors can be added to minimize this attack. 

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