Metals relative corrosive rates

Most galvanic corrosion processes are sensitive to the relatively exposed areas of the noble (cathode) and active (anode) metals. The corrosion rate of the active metal is proportional to the area of exposed noble metal divided by the area of exposed active metal. A favorable area ratio (large anode, small cathode) can permit the coupling of dissimilar metals. An unfavorable area ratio (large cathode, small anode) of the same two metals in the same environment can be costly. 

Tables 2.3 through 2.5 give general corrosion-resistance ratings of different materials. Table 2.3 lists various metals and Table 2.4 gives ratings for various nonmetals. Table 2.5 gives typical corrosion rates of steel and zinc panels exposed to the atmosphere in various locations about the U.S. Figure 2.1 also illustrates relative corrosion rates of steel and zinc in major areas of the world.

Relative Corrosion Rates of Metals (Tests made within the complex level in a pilot-plant reactor)... 

TABLE 9.19. Relative Corrosion Rates of Metals in 50% NaOH... 

Another factor affecting the relative corrosive rate resulting from rain is the orientation of the metal surface. In areas of heavy industrial pollution, skyward-facing metallic surfaces benefit from rain. In those areas where dry deposition is considerably greater than wet deposition of sulfur pollutants, the washing effect of rain predominates, and the corrosion rate is reduced. In areas having less pollution the situation is reversed and the corrosive action of the rain predominates. 

One of the principal reasons for failure due to reaction with the service environment is the relatively complex nature of the reactions involved. Y"et, in spite of all the complex corrosion jargon, whether a metal corrodes depends on the simple elec trochemical cell set up by the environment. This might give the erroneous impression that it is possible to calculate such things as the corrosion rate of a car fender in the spring mush of salted city streets. Dr. M. Pourbaix has done some excellent work in the application of thermodynamics to corrosion, but this cannot yet be applied direc tly to the average complex situation. 

Critical Humidity—the relative humidity (RH) at and above which the atmospheric corrosion rate of a metal increases significantly. 

Critical relative humidity The primary value of the critical relative humidity denotes that humidity below which no corrosion of the metal in question takes place. However, it is important to know whether this refers to a clean metal surface or one covered with corrosion products. In the latter case a secondary critical humidity is usually found at which the rate of corrosion increases markedly. This is attributed to the hygroscopic nature of the corrosion product (see later). In the case of iron and steel it appears that there may even be a tertiary critical humidity . Thus at about 60% r.h. rusting commences at a very slow rate (primary value) at 75-80% r.h. there is a sharp increase in corrosion rate probably attributable to capillary condensation of moisture within the rust . At 90% r.h. there is a further increase in rusting rate corresponding to the vapour pressure of saturated ferrous sulphate solution , ferrous sulphate being identifiable in rust as crystalline agglomerates. The primary critical r.h. for uncorroded metal surfaces seems to be virtually the same for all metals, but the secondary values vary quite widely. 

Rainfall, besides wetting the metal surface, can be beneficial in leaching otherwise deleterious soluble species and this can result in marked decreases in corrosion rate . A recent survey of rainfall analyses for Europe has shown that, with the exception of the UK, the acidity and sulphate content of rainfall markedly increased in the period 1956 to 1966, pH values having fallen by 0 05 to 0-10 units per ann. The exception of the UK may be due to anti-pollution measures introduced in this period. However, even in the UK a pH of 4 is not uncommon for rainfall in industrial areas. The significance of electrolyte solution pH will be discussed in the context of corrosion mechanisms. The remaining cases of electrolyte formation are those in which it exists in equilibrium with air at a relative humidity below 100%. 

Because cast iron components are normally very heavy in section, the relatively low rates of attack associated with atmospheric corrosion do not constitute a problem and little work has been carried out on the phenomenon. A summary of some of the data available is given in Table 3.42. The most extensive work in this field was initiated by the A.S.T.M. in 1958 and some of the results produced by these studies are quoted in Table 3.43. It will be noted that there is a marked fall in corrosion rate with time for all the metals tested. 

The presence of 2% formic acid in acetic acid has relatively little effect on the corrosion of the metal. Among the impurities added at the 0-2% level to 10% acetic acid, only mercuric chloride caused an appreciable increase in corrosion rate (0-71 mm/y), and all the other additions appeared to inhibit corrosion. 

Direct measurements on metals such as iron, nickel and stainless steel have shown that adsorption occurs from acid solutions of inhibitors such as iodide ions, carbon monoxide and organic compounds such as amines , thioureas , sulphoxides , sulphidesand mer-captans. These studies have shown that the efficiency of inhibition (expressed as the relative reduction in corrosion rate) can be qualitatively related to the amount of adsorbed inhibitor on the metal surface. However, no detailed quantitative correlation has yet been achieved between these parameters. There is some evidence that adsorption of inhibitor species at low surface coverage d (for complete surface coverage 0=1) may be more effective in producing inhibition than adsorption at high surface coverage. In particular, the adsorption of polyvinyl pyridine on iron in hydrochloric acid at 0 < 0 -1 monolayer has been found to produce an 80% reduction in corrosion rate . 

Mechanistically, in approximately neutral solutions, solid state diffusion is dominant. At higher or lower pH values, iron becomes increasingly soluble and the corrosion rate increases with the kinetics approaching linearity, ultimately being limited by the rate of diffusion of iron species through the pores in the oxide layer. In more concentrated solutions, e.g. pH values of less than 3 or greater than 12 (relative to 25°C) the oxide becomes detached from the metal and therefore unprotective . It may be noted that similar Arrhenius factors have been found at 75 C to those given by extrapolation of Potter and Mann s data from 300°C. 

The best known way of lowering the corrosion of iron is by its alloying with chrominm, nickel, and other metals. The corrosion resistance of the corresponding stainless steels is dne to the fact that chrominm is readily passivated. This is a quality that is fonnd even in alloys with relatively low chromium contents. Hence, stainless steels are practically always strongly passivated, and their spontaneous dissolution rates are very low. 

The corrosion rate of Pb02 - often enhanced by mechanical erosion - is relatively high and may be a problem due to the toxicity of lead. Pb02 can be stabilized by modification with, for example, silver, antimony, tin, cobalt oxides (or by alloying of the lead base metal with these metals, respectively) [29]. 

Atmospheric corrosion is the most extended type of corrosion in the World. Over the years, several papers have been published in this subject however, most of the research has been made in non-tropical countries and under outdoor conditions. The tropical climate is typical of equatorial and tropical regions and is characterized by permanently high temperatures and relative humidity with considerable precipitation, at least during part of the year. A high corrosion rate of metals is usually reported for this climate. 

Recent reports [30-31] on the use of atmospheric corrosion sensors based on changes in electrical resistance showed that when there were no contaminants [29], in tests of 100-110 h., corrosion rate was zero or insignificant. These sensors can determine changes in metal thickness lower than one nanometer. However, in the presence of 0.08 ppm of S02 or 20 pg/cm2 of NaCl in the system, changes in thickness where always detected over 75% of relative humidity. Corrosion rate was determined at temperatures of 20, 30 and 40°C and the Arrhenius equation was used to calculate the activation energy of the reactions. This method is very similar to the natural conditions. 

The electrical current flowing between the two metals (the corrosion current) is a direct measure of the corrosion rate, since each mole of zinc that dissolves away releases two moles (2F) of electrons for consumption at a relatively remote site, i.e., the cathode. 

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