Carbon steel corrosion dissolved oxygen

Sodium sulfite is added to the filtered water used in these services to scavenge dissolved oxygen and limit carbon steel corrosion. 

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). 

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 ... 

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.)... 

Early work (53) in in-reactor fuel test loops showed that radiolytic oxygen can be suppressed by maintaining 5 to 10 cm Dg/kg DgO dissolved in the coolant and that operation at pH 10 with lithium hydroxide minimizes deposition of magnetite (Fe304) particles on the fuel sheath surfaces. These conditions minimize corrosion and correspond to a minimum solubility of magnetite. With these coolant conditions the fuel surface remains clean and heat transfer is unimpeded—they are the key to the successful use of carbon steel piping, components, and fittings for the CANDU coolant circuit. A simple and effective chemistry control and coolant purification circuit was developed (54). 

Deaeration is accompanied by some reduction of carbon dioxide content, particularly if the water is acidified before the deaeration process to liberate carbonic acid from the dissolved carbonates. Carbonic acid is corrosive to steel in the absence of dissolved oxygen and more so in its presence [18], but addition of alkali to boiler water limits any corrosion caused by carbon dioxide to the boiler itself by converting dissolved carbon dioxide to carbonates. At prevailing boiler temperatures, however, carbonates dissociate as follows ... 

Surfaces exposed to solutions in the containment may be painted or bare steel. Most painted surfaces or clean steel surfaces in contact with water are not good iodine absorbers. Several studies have shown, however, that the zinc primer used to protect carbon steel in containment from corrosion can absorb iodine effectively from solutions at high pH (pH 9-10). The detailed mechanism of iodine absorption on zinc primer surfaces has not been identified. It has been proposed that the metallic zinc in the coating is oxidised by dissolved oxygen in water to form various divalent, insoluble complexes, and that iodide ion, I, may be incorporated in these complexes. The solubilities of the divalent zine complexes depend on pH and the minimum solubilities are in the range of pH = 9.3 to 9.7. At higher and lower values of pH, reactions of iodide ion with zinc primers would be less effective at mitigating the production of molecular iodine. 

Metals and alloys widiout intrinsic corrosion resistance. Such materials can corrode in otherwise innocuous waters or atmospheres, when dissolved oxygen is present or in which water can be reduced to generate free hydrogen. These metals and alloys usually need to be actively protected. Alloys such as low-carbon steels can be used in thick sections to accommodate the loss of material. 

Back, W.C., Kang, T., Sohn, H.J. and Kho, Y.T. (2001) In situ surface-enhanced Raman spectroscopic study on the effect of dissolved oxygen on the corrosion film on low carbon steel in 0.01 M NaCl solution. Electrochimica Acta, 46, 2321-2325. 

The corrosivity of water to mild steel is related to certain chemical aspects of the water. The more important variables are dissolved oxygen, alkalinity, pH, chloride, sulfate, hydrogen sulfide, carbon dioxide, and ammonia. 

In most cases pressure does not have a large influence on corrosion behavior. However, it can affect the composition of the corrodent fluid and thereby have an influence. A classic example is the difference in corrosion behavior of carbon steel in water in an open and closed vessel [76-78] (Fig. 2). Dissolved oxygen is a major source of corrosion of steel by... 

In a similar manner carbon steel is sensitive to pH. In the pH 4.5-9 range the corrosion rate is governed by dissolved oxygen. (See the following discussion on velocity effects.) Below pH 4.5 the corrosion rate is controlled by hydrogen evolution and above about pH 9, the rate is suppressed by an insoluble film of ferric hydroxide (Fig. 5) [IS]. At very high pH levels, especially at elevated temperatures, steel is susceptible to stress corrosion cracking [2i]. 

---