Corrosion rate definition

Fig. 2-9 Relation between potential and corrosion rate of a plain carbon steel in slowly circulating water. Definition of symbols ...

Terminology The International Standards Organization has recently defined a corrosion inhibitor as a chemical substance which decreases the corrosion rate when present in the corrosion system at a suitable concentration, without significantly changing the concentration of any other corrosive agent. This last point is significant since it excludes chemicals employed for deaeration or pH control from the definition of a corrosion inhibitor. On the other hand, it should be noted that the inhibitor is .. . present in the corrosion system. . . , and thus arsenic when added to brasses to prevent dezin-cihcation may be classified as an inhibitor. 

If the definition of TOW established by ISO is used (TOW-ISO), a linear relationship between time and TOW is obtained, in spite of the different possible changes in corrosion rate caused by changes in the nature of TOW. It has to be remarked that it is not the same effect on corrosion rate caused by a heavy rain than dew, fog or water adsorption, so for the same interval of relative humidity (80-100%), notable changes in nature of TOW-ISO and consequently in corrosion rate could take place. 

The metallographic analysis of the reactors with the optical microscope shows not worth mentioning trace of corrosion in the high temperature section of the reactor The elemental analysis of the exposed surface revealed a layer reach in Ni and Fe oxides and depleted in Cr and Mo. These changes affect only a thin layer. In account to the long exposure time of more than 1000 h and the high (up to 0.2 mol/kg) HCl concentration, corrosion rates of alloy 625 in high temperature - low-density supercritical water solutions are definitely low. For such HCl and Oj solutions Ni-base alloys, similar to alloy 625, can be used. 

Carbon Steel and Cor-Ten A Steel. The corrosion rates for Cor-Ten A steel at the four sites are shown in Table I. Corrosion rates for carbon steel at the North Carolina, District of Columbia, and New Jersey sites are lower than those for Cor-ten A during 1-month exposures, equal to Cor-ten A during 3-month exposures, and higher than Cor-ten A for 1-year exposures. Corrosion rates for carbon steel at the New York site are lower than Cor-ten A during the 1- and 3-month exposures and equal to Cor-ten A after 1 year of exposure. These results indicate, relative to carbon steel, that Cor-ten A must corrode a definite amount before producing a corrosion film of sufficient thickness to exhibit protective properties. 

Corrosion rate was evaluated with respect to, 1) flux of pollutants (sulfur oxides, nitrogen oxides, oxidants, and particles) to the steel during both wet and dry periods, 2) temperature, and 3) exposure history. Different definitions of when the steel was wet were evaluated to determine the most likely "critical relative humidity." Non-linear multiple regression techniques were used to determine the statistical significance of each factor and develop a theoretically consistent environmental damage function. 

The ohmic drop exerts a sensible influence on the evaluation of the electrochemical parameters as well as on the definition of the reaction scheme that is most suitable for describing the behaviour of a metal in a given environment. It also determines the success of many operations, such as cathodic protection by means of sacrificial anodes or impressed current and corrosion rate monitoring. 

Corrosives are the largest class of chemicals used by industry, so it stands to reason that they would frequently be encountered in transportation and at fixed facilities. DOT Class 8 materials are corrosive liquids and solids. There are no DOT subclasses of corrosives. There are, however, two types of corrosive materials found in Class 8 acids and bases. Acids and bases are actually two different types of chemicals that are sometimes used to neutralize each other in a spill. They are grouped together in Class 8 because the corrosive effects are much the same on tissue and metals, if contacted. It should be noted, however, that the correct terminology for an acid is corrosive and for a base is caustic. DOT, however, does not differentiate between the two when placarding and labeling. The DOT definition for a corrosive material is a liquid or solid that causes visible destruction or irreversible alterations in human skin tissue at the site of contact, or a liquid that has a severe corrosion rate on steel or aluminum. This corrosive rate on steel and aluminum is 0.246 inches per year at a test temperature of 131 F. ... 

For polycrystalline metals, the dissolution rate of the various crystallographic orientations is different. Grain boundaries or precipitations will also show a different corrosion rate. There is no accepted definition for what constitutes uniform corrosion. A possible definition could be that the variation of thickness loss all over the surface should not he greater than 5%. 

A corrosion inhibitor in general terms is a chemical substance that when added in a small amount to an environment effectively reduces the corrosion rate of a metal or alloy exposed to the corrosive environment. A more precise definition of an inhibitor is not possible because of the number of mechanistic and/or chemical considerations when classifying corrosion inhibitors. 

In several cases, materials for combined erosive and corrosive conditions have been evaluated on the basis of separate erosion and corrosion studies and data, with the consequence that the synergistic effects are left out of the evaluation. Since one or the other of these effects may be large, the conclusions may be quite wrong. For materials fliat usually are passive due to a dense oxide film, such as stainless steels, Wc is by definition very low. But since sand erosion more or less destroys the passive film, the corrosion rate increases strongly and may reach very high values, i.e. the contribution of Wce may be particularly high for these materials. The other synergy effect, Wec, is most pronounced for ceramic-metallic materials in which the metallic phase has inferior corrosion resistance, e.g. for a cemented carbide with a metallic phase of cobalt (WC-Co). 

We will need definitive evidence of the dosage vs. chloride level to achieve a given (low) corrosion rate. 

The polarization term that controls the corrosion rate of many metals in deaerated water and in nonoxidizing acids is hydrogen overpotential. In accord with the previously discussed definition of polarization, hydrogen overpotential is the difference of potential between a cathode at which hydrogen is being evolved, ( )measured, 3nd a hydrogen electrode at equilibrium in the same solution that is. 

Examples of metals that are passive under Definition 1, on the other hand, include chromium, nickel, molybdenum, titanium, zirconium, the stainless steels, 70%Ni-30% Cu alloys (Monel), and several other metals and alloys. Also included are metals that become passive in passivator solutions, such as iron in dissolved chromates. Metals and alloys in this category show a marked tendency to polarize anodicaUy. Pronounced anodic polarization reduces observed reaction rates, so that metals passive under Definition 1 usually conform as well to Definition 2 based on low corrosion rates. The corrosion potentials of metals passive by Definition 1 approach the open-circuit cathode potentials (e.g., the oxygen electrode) hence, as components of galvanic cells, they exhibit potentials near those of the noble metals. 

Fig. 6.1), so that the current density and accompanying corrosion rate correspond to the low value of (passive- This process is called anodic protection (see Section 13.9) because the current flow is in the direction opposite to that which is used in cathodic protection. Whereas cathodic protection can, in principle, be applied to both passive and nonpassive metals, anodic protection is applicable only to metals that can be passivated when anodically polarized (see Definition 1, Section 6.1). 

An inhibiting mechanism similar to that for nontransition metals in contact with passivators probably also applies to steel in concentrated refrigerating brines (NaCl or CaCy to which chromates are added as inhibitors (approximately 1.5-3. Og Na2Cr207/liter adjusted with NaOH to form CrOi ). In the presence of so large a Cr concentration, passivity of the kind discussed under Definition 1 (Section 6.1) does not take place. The reduction in corrosion rate is not as pronounced as when chlorides are absent [14] (see Table 17.1), and any reduction that occurs apparently results from formation of a surface diffusion barrier of chromate reduction products and iron oxides. Chromates are not adequate inhibitors for the hot concentrated brine solutions that, in the past, were sometimes mistakenly proposed as antifreeze solutions for engine cooling systems. 

One definition of effective cathodic protection is to depress the potential of the cathodes to the level of the anodes, thus stopping current from flowing between anodic and cathodic areas (Mears and Brown, 1938). This works because cathodes are more easily polarized (potential shifted) than anodes. We saw this phenomenon in Section 4.11 where the effect of an external current on the half cell potential allows us to calculate the corrosion rate. 

Anderson and Reinhard (1939) observed definite differences in the corrosion rates of sheet zinc exposed during different periods of the day. Specimens were exposed for 8 hours per day and were stored in dry, unheated rooms for the other 16 hours. The test was planned so that a different group of specimens was exposed during each 8-hour period 08 15 to 16 15, 16 15 to (X) 15, and 00 15 to 08 15. The test continued for 6 years, and thus each set was exposed to the atmosphere of Palmerton, Pennsylvania, for a cumulative... 

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