Uniform Corrosion Rates

Uniform corrosion of metals can usually be predicted from laboratory tests or experience. Corrosion allowances, which will require thicker metal, can be called for in the design of equipment when uniform corrosion rates are expected. 

Uniform corrosion in metals can usually be predicted from lab tests or experience. Corrosion allowances that require thicker metal can be called for in the design of equipment when uniform corrosion rates are expected. However, uniform corrosion is often not the worst thing that can happen to materials. The most important materials failure to avoid in the design of metal equipment is sudden catastrophic failure. This occurs when the material fractures under impulse instead of bending. Catastrophic failure can cause complete destruction of piping or equipment and can result in explosions, huge spills, and consequent fires. Some of the more common types of catastrophic failure are described in the following sections. 

Prevention—Hydrogen embrittlement is prevented or minimized by (1) reducing the uniform corrosion rate on a metal surface (by coating/painting the metal surface, for example), to decrease the rate of hydrogen evolution on the metal surface (2) baking the metal (hydrogen evolution is an... 

The characteristics of individual forms of corrosion are taken into consideration by providing appropriate corrosion specimens. Welded coupons having the surface quality of the material used later in practice are sufficient for determining uniform corrosion rates and acquiring general information on the type of local corrosion. Resistance to crevice corrosion can be determined by using specimens as described in ASTM G 78-83. Conditions of heat transfer can be simulated by using hot-wall/cool-wall specimens under temperature-controlled conditions. 

For copper, the critical rate of 28.5 gmd (285 mdd) is much higher than the uniform corrosion rates in aerated water and 3% NaCl (0.4-1.5gmd, 4-15 mdd) hence, the fatigue life of copper is observed to be about the same in air as in fresh and saline waters (Table 8.5). 

The Pb-Bi eutectic is used as a coolant in the LMRs installed in submarine factory number 601. In the absence of information on the performance of this material in sea water, the model assumes the same corrosion rate as Pb. Heiser and Soo [23] report uniform corrosion rates of 0.008 to 0.013 mm-a, and a BCR for Pb of 0.01 mm-a is therefore not unreasonable. There is clearly some uncertainty over the performance in sea water of the Pb-Bi eutectic compared with pure Pb. However, until further data becomes available on the former, it is considered a representative rate to use in the model at this stage. 

Ferrous alloys have demonstrated varying compatibility with liquid Pb, Pb-Li and Pb-Bi eutectics. In general, ferritic stainless steels such as HT-9 and Fe-9Cr-lMo exhibit superior corrosion resistance in these envirorunents relative to austenitic stainless steels like 316. While exposed to flowing (0.351/min.) Pb-17Li at 550°C [1], HT-9 possessed a uniform corrosion rate of 20 um/year. The dissolution rate activation energy was estimated to be 92.5 kJ/mol. Ferritic... 

The uniform corrosion rates for zinc are not greatly affected by the purity of zinc 98.5 and 99.99% zincs behave similarly in many conditions. This is especially true in open atmospheres, where sufficient oxygen is present to prevent polarization by hydrogen. Some alloying elements increase the corrosion resistance of zinc significantly. 

The impedance method consists in measuring the response of an electrode to a sinusoidal potential modulation of small amplitude (typically 5-10 mV) at different frequencies. The ac modulation is superimposed either onto an applied anodic or cathodic potential or onto the corrosion potential. Another possibility is to modulate the current and to measure the potential. Impedance measurements as a function of modulation frequency are commonly referred to in the literature as electrochemical impedance spectroscopy, abbreviated EIS. Among the different transient methods discussed in this chapter, EIS is most widely used in corrosion studies. It serves for the measurement of uniform corrosion rates, fortheelucidationofreactionmechanisms, for the characterization of surface films and for testing of coatings. 

The presence of chlorides or sulfates leads to the formation of the hydroxy-chlorides A1(0H)2C1 and Al(OH)Cl2 or of hydroxy-sulfates Al(0H)x(S04)y. These compounds dissolve in acidic environments and the resulting corrosion products are carried away by run-off water. However, even in a polluted atmosphere, the uniform corrosion rate of aluminum remains generally small. On the other hand, in presence of chloride pollution, damage by pitting may occur. 

In-situ testing is also performed in the field to determine the effectiveness of a water treatment system, the performance of difference alloys, and the effects of var5dng operating parameters such as flow rate and temperature. Uniform corrosion rates, t5fpes of corrosion, and pitting tendencies are identified with in-situ testing. 

The following standardized test methods are relevant for determining a uniform corrosion rate of a material in a specific test environment ... 

Other factors may influence uniform corrosion rates, such as the development of oxide layers on a metallic surface. In reality, such layers limit the corrosion rate by affecting diffusion and imparting concentration control. 

The salinity of open ocean water at the surface typically varies from 32-37.5 %. Generally, this degree of variation does not alter metal corrosion rates. Some of the more isolated seas, such as those shown in Table 5, can have large variations in salinity, which may influence metal corrosion rates. Salinity variations can affect a metal s uniform corrosion rate. For example, in areas where there are high evaporation rates, the salinity can be very high. Conversely, at the mouth of a river, the salinity is more dilute than in the open ocean. 

Uniform corrosion of MMC matrices can be expected in environments that attack the matrix metal uniformly. In continuous-fiber MMCs, however, fibers will be left in relief as the matrix corrodes, whereas, in particulate MMCs, reinforcement particles fall free as they are undercut. Uniform corrosion rates of MMCs may be greater than that of the monolithic matrix alloys, due to galvanic action between the matrix and reinforcement constituents. 

MMCs should be exposed to environments of interest and then inspected. If corrosion is uniform, corrosion rates can be assessed using recommendations in ASTM G I. Electrochemical measurements can also be used to determine uniform corrosion rates. Refer to ASTM G 102, Calculation of Corrosion Rates and Related Information from Electrochemical Measurements. Note that when determining matrix corrosion rates, the corrosion current should be normalized with respect to the matrix area if reinforcements are inert... 

Metal loss is determined from resistance change of a corroding metal element. Uniform corrosion rates are calculated from resistance reading. Does not measure localized corrosion. 

Coupon Testing Long-term duration, 30 to 90 days Average uniform corrosion rate Poor Any Uniform and localized attack Simple... 

The major problem in using Mg-air primary batteries is the low Coulombic efficiency of the traditional Mg plate electrode and the high level of polarization during the discharge process. Moreover, the electrol3Ae for Mg-air batteries must satisfy the objectives of low anodic polarization, a uniform corrosion rate of Mg, and fast sedimentation of the final anodic product (Mg(OH)2) in the electrol3Ae. 

Uniform surface corrosion, i.e. corrosion at a nearly uniform corrosion rate over the entire surface, is usually less problematic from an operational point of view. This factor can be taken into account in the structural element design in the form of an anticorrosive additive and can be controlled in many structural elements by means of regular wall thickness measurements, e.g. by ultrasonic means. Much more difficult problems result from local corrosion types such as pitting corrosion and stress corrosion cracking (SCC). The corrosion types are difficult to control and can rapidly lead to failure of structural elements after only a low level of mass loss. Damage from such corrosion types are rarely predictable and not only cause considerable losses in economic terms but also entail risks to safety and environmental protection. This applies in particular to system elements that must function under pressure. 

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