Uniform corrosion steels

This criterion is derived from the fact that the free corrosion potential in soil is generally I/cu Cuso4 -0-55 V. Ohmic voltage drop and protective surface films are not taken into consideration. According to the information in Chapter 4, a maximum corrosion rate for uniform corrosion in soil of 0.1 mm a can be assumed. This corresponds to a current density of 0.1 A m l In Fig. 2-9, the corrosion current density for steel without surface film changes by a factor of 10 with a reduction in potential of about 70 mV. To reduce it to 1 jum a (0.14 V would be necessary. The same level would be available for an ohmic voltage drop. With surfaces covered with films, corrosion at the rest potential and the potential dependence of corrosion in comparison with act contrary to each other so that qualitatively the situation remains the same. More relevant is... 

Fig. 1.46 Localised attack due to discontinuity in millscale or a deposit at a steel surface, (a) Uniform corrosion, (b) localised attack at a discontinuity in millscale, (c) increase in rate due to increase in the supply of oxygen and (d) crevice formed by deposit. (Arrows pointing downwards represents the cathodic current, and arrows pointing upwards the anodic current)...

In atmospheric exposure 18% Ni maraging steel corrodes in a uniform manner , and becomes completely rust covered. Pit depths tend to be more shallow than for the low-alloy high-strength steels. Atmospheric corrosion rates in industrial (Bayonne, New Jersey) and marine (Kute Beach, North Carolina) atmospheres are compared with those for low-alloy steel in Figs. 3.29, 3.30 and 3.31. The corrosion rates drop substantially after the first year or two and in all cases the rates for maraging steel are about half the corrosion rate for HY80 and AISI 4340 steels. 

Dissolution of steel or zinc in sulfuric or hydrochloric acid is a typical example of uniform electrochemical attack. Steel and copper alloys are more vulnerable to general corrosion than other alloys. Uniform corrosion often results from atmospheric exposure (polluted industrial environments) exposure in fresh, brackish, and salt waters or exposure in soils and chemicals. The rusting of steel, the green patina on copper, tarnishing silver and white mst on zinc on atmospheric exposure are due to uniform corrosion.14... 

Protective passive films similar to that of stainless steel, for example, result in uniform corrosion because of the mobility of active sites that passivate readily. Corrosion products and/or passive films are characteristic of numerous electrochemical corrosion of alloys. A film is protective depending on coverage capacity, conductivity,... 

Uniform corrosion usually occurs in fairly aggressive environments that attack the whole surface. Examples include carbon steel in seawater or acids, or aluminum alloys in strong alkali. The rate of metal loss is usually rather high, but, because it is distributed over the whole surface, the performance can usually be predicted, and managed with corrosion allowances, in most situations. Thus, sheet steel piling is often used in seawater without any corrosion protection, the corrosion rate of around 0.1 mm/yr, coupled with the relatively thick steel sections, giving an acceptable life. 

Very often the long experience with chemicals operating as corrosion inhibitors, e. g. in the oil field, gas or petroleum industry, is taken as an example for the successful use of corrosion inhibitors for many decades. This undoubtedly is true and the overwhelming majority of literature on corrosion inhibitors deals with the effects of inhibitors on uniform corrosion, e. g. of steel in acidic or neutral solutions, where they can be classified into [2] a) adsorption inhibitors, acting specifically on the anodic or on the cathodic partial reaction of the corrosion process or on both reactions (mixed inhibitor), b) JUm-forming inhibitors, blocking the surface more or less completely, and c) passivators, favouring the passivation reaction of the steel (e. g. hydroxyl ions). 

For materials like stainless steels, the mechanisms are quite different. Corrosion resistance in stainless steels is provided by a passive film that acts as a barrier between the alloy and the water. The passive film is a continuous, non-porous and insoluble film, which, if broken under normal conditions, is self-healing. Due to these characteristics, the uniform corrosion of stainless steels is usually very low and the major risk is pitting corrosion. The pitting corrosion risk of stainless steels is influenced not only ly the composition of the alloy and by water quality but also by service conditions, quality of the material and quality of the installation (fitting, soldering conditions, etc.). 

Parallel use of the Pourbaix diagram and overvoltage curves is sometimes useful when evaluating how typical and stable the uniform corrosion is under different conditions. Let us consider a case with iron or unalloyed steel in two different environments ... 

Crevice corrosion is also a form of localized corrosion. It amounts to locally intensified corrosion in crevices and thus results exclusively from the design of a specific component (Fassler 1975). In alloyed steel, it usually occurs as nonuniform or uniform corrosion, and in stainless steels, it can also occur as pitting corrosion. It can be regarded as a variation of pitting corrosion. 

Experience tells us that a pile of nails left out in the yard rusts without contact with another metal (Figure 13.6). This implies that the second half-cell in this backyard redox reaction must involve a nonmetal. In uniform corrosion, rust can cover the surface of iron or steel. The second electrode is a second region of the iron itself, located some distance away from the first spot. Ions that can conduct current help facilitate this process, so when the chloride ions of salt are present the rate of rusting is enhanced. 

This is the most common form of corrosion when metal dissolution takes place uniformly (not localized) over the entire exposed surface (e.g., rusting). Uniform corrosion can be seen in steel pipes, beams, offshore drilling platforms, heat exchanger tubes, knives, forks, and spoons. 

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