Carbon steel pitting

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). 

In this same amine service it was found that carbon steel pits much more readily if it has not been stress relieved. 

Stainless steels and aluminum and its alloys are particularly susceptable to pitting. Carbon steels are more resistant to pitting than stainless steels. Most failure of stainless steels occurs in neutral-to-acid chloride solutions. Aluminum and carbon steels pit in alkaline chloride solutions. 

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. 

Figure 6.6 Clustered sulfate-reducer pits on a carbon steel tank bottom.

Figure 6.10 A perforated carbon steel pipe at a weld-backing ring. The gaping pit was caused by sulfate-reducing bacteria (see Case History 6.1).

Figure 7.19 Pits on a carbon steel check valve. Note how pits intersect to form areas of jagged metal loss. A steel prohe tip is in the photo. (Magnification 7.5x.)...

A tank with a fixed cover of plain carbon steel for storing 60°C warm, softened boiler feed water that had a tar-pitch epoxy resin coating showed pits up to 2.5 mm deep after 10 years of service without cathodic protection. Two separate protection systems were built into the tank because the water level varied as a result of service conditions. A ring anode attached to plastic supports was installed near the bottom of the tank and was connected to a potential-controlled protection rectifier. The side walls were protected by three vertical anodes with fixed adjustable protection current equipment. 

The protection current requirement is determined mainly by the uncoated surfaces of the stainless steel whose protection potential is a few tenths of a volt more positive than that of the plain carbon steel, to avoid pitting (sec Section 2.4). The protection current requirement for the turbine section is about 10 A so that the plate anodes are only loaded to about 1 A. 

In a few solutions such as distilled, tap, or other fresh waters, the stainless steels pit but it is of a superficial nature. In these same solutions carbon steels suffer severe attack. 

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. 

Fig. 1.57 Electrochemical reactions that occur when a pit is initiated at sulphide inclusion in a carbon steel (after Wranglen )...

Wranglen considers that sulphide inclusions are responsible for the initiation of attack in both carbon steels and stainless steels, and on this basis he has provided a detailed exposition of the pitting of a carbon steel at an inclusion of MnS when the steel is immersed in an oxygenated chloride solution (Fig. 1.57). The reactions of significance are given in the diagram, but certain features of the mechanism are of interest since they illustrate the complexity of the process. 

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. 

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. 

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. 

Kitchen steam generators to provide live steam for steam tables are widely available. They tend to be very small, perhaps only of 150 to 250 lb/hour generating capacity. However, because they typically receive 100% cold, city supply as a FW source, and where the boiler vessels are constructed of carbon steel, oxygen pitting and subsequent boiler failure can occur rapidly. 

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. 

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. 

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. 

A source of doubt in such analyses is whether the depths of the pits grown electrochemically are representative of those expected under natural conditions and therefore appropriate to extrapolate to longer times in predictive models. The data shown for pitting of carbon steel, sketched in Fig. 28, show that they are not. Clearly, growth is accelerated under potentiostatic electrochemical conditions, and the extrapolation of pit depths seriously overestimates the predicted pit depths after long exposure times. This is not surprising, since the use of a... 

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. 

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