Carbon steel oxygen corrosion

Fig. 24. Corrosion rates of carbon steel as a function of oxygen concentration at cation conductivity values of ( ...

Figure 5.2 Schematic of carbon steel corrosion rate versus exposure time in a typical oxygenated cooling water. Note how the average corrosion rate decreases with time and converges to CR at t (the minimum exposure time to get reproducible results).

Carbon steel heat exchangers, cast iron water boxes, screens, pump components, service water system piping, standpipes, fire protection systems, galvanized steel, engine components, and virtually all non-stainless ferrous components are subject to significant corrosion in oxygenated water. 

Carbonic acid is corrosive to steel even if oxygen is absent. Corrosion is accelerated greatly by the presence of dissolved oxygen, however. 

Electrical conductivity is of interest in corrosion processes in cell formation (see Section 2.2.4.2), in stray currents, and in electrochemical protection methods. Conductivity is increased by dissolved salts even though they do not take part in the corrosion process. Similarly, the corrosion rate of carbon steels in brine, which is influenced by oxygen content according to Eq. (2-9), is not affected by the salt concentration [4]. Nevertheless, dissolved salts have a strong indirect influence on many local corrosion processes. For instance, chloride ions that accumulate at local anodes can stimulate dissolution of iron and prevent the formation of a film. Alkali ions are usually regarded as completely harmless, but as counterions to OH ions in cathodic regions, they result in very high pH values and aid formation of films (see Section 2.2.4.2 and Chapter 4). 

In natural waters, cold-worked commercial carbon steels of the same composition corrode at more or less the same rate as annealed steels, presumably because the corrosion rate in this case is controlled by the diffusion of oxygen. Unprotected carbon steels are sometimes exposed to natural waters, and it is this latter situation which is of greater practical importance than the behaviour of steels in acids, since steels should never be used in these environments unless they are protected. 

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. 

The pre-boiler equipment, consisting of feedwater heaters, feed pumps and feed lines, is constructed of a variety of materials, including copper, copper alloys, carbon steel, and phosphor bronzes. To reduce corrosion, the makeup and condensate must be at the proper pH level and free of gases such as carbon dioxide and oxygen. The optimum pH level is that which introduces the least amount of iron and copper corrosion products into the boiler cycle. This optimum pH level should be established for each installation. It generally ranges between 8.0 and 9.5... 

One of the major problems encountered in the storage and transport of anhydrous liquid ammonia is the stress-corrosion-cracking (SCC) of carbon steel equipment. Cracks most often occur at the weld joints, where the leftover stress is at a maximum. The leftover stress is that which remains even after heat treatment. The hardness of the material and the presence of impurities and oxygenates in ammonia aggravate SCC88. 

Data is shown in Figs. 7a and 7b for oxygen reduction on carbon steel in room temperature 0.6M NaCl. iL increases with co0 5 as predicted. Hence if the corrosion rate is determined by the mass transport of oxygen to the disk surface to support oxygen reduction, then the corrosion rate will increase as a function of the rotation rate, co, raised to the 0.5 power and linearly with dissolved oxygen concentration. The diffusion boundary layer thickness, 8d, may be calculated from Fick s first law after iL is determined. Recall that 8 = nFDCJiL for one dimensional diffusion at the steady state. This leads to the following expression for the diffusional boundary layer thickness ... 

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 ... 

As mentioned previously, oxygen controls the corrosion rate in the neutral pH range, if the oxygen is less than 1 ppm, the penetration rate in carbon steel or cast iron will be less than 1 mpy (0.025 mm/y) at room temperature, provided no corrosive bacteria are present. If corrosion-inducing bacteria are present, treatment with a biocide such as chlorine is imperative. In theory, because freshwater can be treated, carbon steel exchanger tubes can be used. However, control of water treating equipment is sufficiently difficult and... 

Clean condensate, free of solids and gases (including oxygen), is relatively noncorrosive and can be handled in carbon steel with a minimal corrosion allowance. 

There is some question about the need to stress relieve carbon steel in a carbonate solution, which normally contains corrosion inhibitors. Oxygen is usually added to maintain the inhibitor, which is often vanadium pentoxide in the active (oxidized) state otherwise, SCC can occur. 

Oxygen-deficient atmospheres can develop in closed containers by a number of seemingly harmless mechanisms. Corrosion inside of a closed container is such a mechanism. The typical corrosion rate for carbon steel in very moist air is about 0.005 in.fyr and is about first order with respect to the oxygen concentration. If the corrosion reaction can be approximated by the reaction,... 

At low water contents, HF has a very low corrosion rate on mild carbon steel, but as the water content rises above 5%, the corrosion rate rapidly increases. Corrosion is anticipated wherever there is a possibility of acid or acid-hydrocarbon mixtures in contact with water. Copper, silver, and platinum are resistant to corrosion from all concentrations of HF, with the exception that copper and silver are attacked in the presence of sulfur compounds and oxygen. Lead is useful only if the water content is above 35%. Finally, stainless steel has poorer corrosion resistance to HF than carbon steel. 

Polarization occurs because of ion concentration buildup near the anode and/or cathode. Once the ion concentration reaches saturation, corrosion essentially stops. Polarization can occur when (1) Hydrogen ions concentrate at an active cathode in the absence of a cathodic depolarizer. Dissolved oxygen acts as a cathodic depolarizer. (2) Metal ions saturate the electrolyte around an anode. (Soluble Fe++ may saturate the anode, perhaps as the result of the precipitation of an insoluble iron salt, inhibiting the diffusion of Fe++. For example, insoluble surface compounds such as carbonate scales in a fresh water often occur on carbon steel.)... 

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