Corrosion by oxygen

however, the water contains dissolved oxygen, as it usually does if it is or has been in contact with air, the oxygen absorption reaction 16.4 becomes a likely cathode reaction  

In neutral water or even 1 mol alkali, for reaction 16.4, better written for basic media as reaction 16.5, is still quite positive (-1-0.815 and -fO.401 V for 0.1 MPa O2 at pH 7 and 14, respectively, or about 0.01 V less for air at 0.1 MPa). Thus, oxygen absorption remains an important cause of aqueous metallic corrosion, even in alkaline media, although it is clear from reactions 16.4 and 16.5 that it is favored by low pH. 

Oxygen absorption, then, is the usual complementary cathodic reaction for the anodic dissolution of metals other than the most reactive ones (Li, Na, K, Ca, etc.), in near-neutral aerated water. Regardless of whether the cathodic reaction is 16.3 or 16.5, however, the following observations regarding bimetallic corrosion apply  

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When a corrosive environment contains only small concentrations of an oxidizing agent, the corrosion rate is often limited by its transport. This behavior is typically found for corrosion by oxygen in neutral media or corrosion by protons in weakly acidic environments. 

The kinetics of reactions in which a new phase is formed may be complicated by the interference of that phase with the ease of access of the reactants to each other. This is the situation in corrosion and tarnishing reactions. Thus in the corrosion of a metal by oxygen the increasingly thick coating of oxide that builds up may offer more and more impedance to the reaction. Typical rate expressions are the logarithmic law,... 

Vanadium is resistant to attack by hydrochloric or dilute sulfuric acid and to alkali solutions. It is also quite resistant to corrosion by seawater but is reactive toward nitric, hydrofluoric, or concentrated sulfuric acids. Galvanic corrosion tests mn in simulated seawater indicate that vanadium is anodic with respect to stainless steel and copper but cathodic to aluminum and magnesium. Vanadium exhibits corrosion resistance to Hquid metals, eg, bismuth and low oxygen sodium. 

The use of neutralising amines in conjunction with an oxygen scavenger—metal passivator improves corrosion control in two ways. First, because any acidic species present is neutralized and pH is increased, the condensate becomes less corrosive. Second, most oxygen scavenger—passivators react more rapidly at the mildly alkaline conditions maintained by the amine than at lower pH levels. For these reasons, this combination treatment is gaining wide acceptance, particularly for the treatment of condensate systems that are contaminated by oxygen. 

As discussed above, deposits can cause accelerated localized corrosion by creating differential aeration cells. This same phenomenon occurs with a biofilm. The nonuniform nature of biofilm formation creates an inherent differential, which is enhanced by the oxygen consumption of organisms in the biofilm. 

Reduction of oxygen is one of the predominant cathodic reactions contributing to corrosion. Awareness of the importance of the role of oxygen was developed in the 1920s (19). In classical drop experiments, the corrosion of iron or steel by drops of electrolytes was shown to depend on electrochemical action between the central relatively unaerated area, which becomes anodic and suffers attack, and the peripheral aerated portion, which becomes cathodic and remains unattacked. In 1945 the linear relationship between rate of iron corrosion and oxygen pressure from 0—2.5 MPa (0—25 atm) was shown (20). 

The crevice geometry and normally occurring corrosion combine to produce accelerated attack in the shielded region, a so-called autocat-alytic process. Initially, corrosion in oxygenated water of near neutral pH occurs by Reactions 2.1 and 2.2 ... 

Oxygen has a dual role in corrosion of copper. It stimulates attack by an interaction at the cathode, and it retards corrosion by producing a protective corrosion-product layer. High velocity can disturb the corrosion-product layer, increasing attack. 

Dissolved oxygen is the enemy of anaerobic bacteria. Naturally occurring corrosion consumes oxygen. Without convective replenishment, oxygen-depleted zones form and corrosion by anaerobes is usually worse. 

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). 

It is a consequence of the action of different pH values in the aeration cell that these cells do not arise in well-buffered media [4] and in fast-flowing waters [5-7]. The enforced uniform corrosion leads to the formation of homogeneous surface films in solutions containing Oj [7-9]. This process is encouraged by film-forming inhibitors (HCOj, phosphate, silicate, Ca and AP ) and disrupted by peptizing anions (CP, SO ") [10]. In pure salt water, no protective films are formed. In this case the corrosion rate is determined by oxygen diffusion [6,7,10]... 

By removing oxygen completely, corrosion by this gas is eliminated. It can be achieved by the addition of sodium sulfite or hydrazine, which reacts with oxygen. The reaction product will not normally cause any problems. 

If the instrument indicates current surge in an air-free system, it generally implies hydrogen sulfide contamination, but the galvanic probe is usually best suited to detect corrosion influenced by oxygen contamination. 

The irons are most useful in environments containing a plentiful supply of oxygen or oxidising agents anaerobic or reducing conditions may lead to rapid corrosion. Physical effects such as abrasion or sudden dimensional changes induced by temperature fluctuations may rupture the film and allow corrosion to take place. The iron will also be subject to corrosion by solutions containing anions, such as those of the halides, which can penetrate surface films relatively readily. 

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