Metal corrosion evaluation

Bearing metal corrosion is evaluated by the Mirrlees corrosion test in which test coupons are suspended in the candidate oil for 100 h at 140 C. The procedure evaluates the tendency of a lubricant to cause grain boundary attack and incipient corrosion on small specimens of ... 

Wachter, A. and Treseder, R. S., Corrosion Testing Evaluation of Metals for Process Equipment, Chemical Engineering Progress, Vol. 43, June 1947, pp. 315-326. 

It is absolutely imperative that all corrosion inhibitor evaluations be made on metal specimens encountered in the field. It had been pointed out above that [27,25] the effective inhibitor concentration on two different oilfield steels (J-55 and L-80) was vastly different under severe conditions. Examples of this abound in the industry. Weldments of carbon steel piping containing a small amount of copper were more difficult to inhibit under relatively mild conditions. 

Natalie, C. A., Evaluation of Uniform Corrosion, Metals Hand book. Ninth Edition, Volume 13, ASM International, Metals Park, OH, pp. 229-230. 

FlOiei Evaluation of techniques for assessing corrosion cracking In dissimilar metal welds Dr D.R. Tice AEA Technology... 

Duration of Test Although the duration of any test will be determined by the nature and purpose of the test, an excellent procedure for evaluating the effect or time on corrosion of the metal and also on the corrosiveness of the environment in laboratory tests has been presented by Wachter and Treseder [Chem. Eng. Pi og., 315-326 (June 1947)]. This technique is called the planned-interval test. Other procedures that require the removal of sohd corrosion products between exposure periods will not measure accurately the normal changes of corrosion with time. 

Tafel Extrapolation Corrosion is an elec trochemical reac tion of a metal and its environment. When corrosion occurs, the current that flows between individual small anodes and cathodes on the metal surface causes the electrode potential for the system to change. While this current cannot be measured, it can be evaluated indirectly on a metal specimen with an inert electrode and an external electrical circuit. Pmarization is described as the extent of the change in potential of an electrode from its equilibrium potential caused by a net current flow to or from the electrode, galvanic or impressed (Fig. 28-7). 

Evaluation of the corrosion potential from Eq. (2-53) is inadmissible when the metal ion, for example, reacts further in strongly complexing solutions ... 

Pourbaix has evaluated all possible equilibria between a metal M and HjO (see Table 1.7) and has consolidated the data into a single potential-pH diagram, which provides a pictorial summary of the anions and cations (nature and activity) and solid oxides (hydroxides, hydrated oxides and oxides) that are at equilibrium at any given pH and potential a similar approach has been adopted for certain M-H2O-X systems where A" is a non-metal, e.g. Cr, CN , CO, SOj , POj", etc. at a defined concentration. These diagrams give the activities of the metal cations and anions at any specified E and pH, and in order to define corrosion in terms of an equilibrium activity, Pourbaix has selected the arbitrary value of 10 ° g ion/1, i.e. corrosion of a metal is defined in terms of the pH and potential that give an equilibrium activity of metal cations or anions > 10 g ion/1 conversely, passivity and immunity are defined in terms of an equilibrium activity of < 10 g ion/1. (Note that g ion/1 is used here because this is the unit used by Pourbaix in the S.I, the relative activity is dimensionless.)... 

In uniform corrosion the superficial or geometrical area of the metal is used to evaluate both the anodic and cathodic current density, although it might appear to be more logical to take half of that area. However, surfaces are seldom smooth and the true surface area may be twice to three times that of the geometrical area (a cleaved crystal face or an electropolished single crystal would have a true surface area that approximates to its superficial area). It follows, therefore, that the true current density is smaller than the superficial current density, but whether the area used for calculating /, and... 

It follows from equation 1.45 that the corrosion rate of a metal can be evaluated from the rate of the cathodic process, since the two are faradai-cally equivalent thus either the rate of hydrogen evolution or of oxygen reduction may be used to determine the corrosion rate, providing no other cathodic process occurs. If the anodic and cathodic sites are physically separable the rate of transfer of charge (the current) from one to the other can also be used, as, for example, in evaluating the effects produced by coupling two dissimilar metals. There are a number of examples quoted in the literature where this has been achieved, and reference should be made to the early work of Evans who determined the current and the rate of anodic dissolution in a number of systems in which the anodes and cathodes were physically separable. 

The thermodynamic phase stability diagrams appear to be preferred by corrosion scientists and technologists for the evaluation of gas-metal systems where the chemical composition of the gaseous phase consisting of a single gas or mixture of gases has a critical influence on the formation of surface reaction products which, in turn, may either stifle or accelerate the rate of corrosion. Also, they are used to analyse or predict the reason for the sequence of formation of the phases in a multi-layered surface reaction product on a metal or alloy. 

Diegel, R. B. and Kelley, K.. Comparative Corrosion Evaluation of Zinc-based and Aluminium Metallized Coatings, Texasgulf (now Falconbridge), Toronto (1981)... 

It is evident from previous considerations (see Section 1.4) that the corrosion potential provides no information on the corrosion rate, and it is also evident that in the case of a corroding metal in which the anodic and cathodic sites are inseparable (c.f. bimetallic corrosion) it is not possible to determine by means of an ammeter. The conventional method of determining corrosion rates by mass-loss determinations is tedious and over the years attention has been directed to the possibility of using instantaneous electrochemical methods. Thus based on the Pearson derivation Schwerdtfeger, era/. have examined the logarithmic polarisation curves for potential breaks that can be used to evaluate the corrosion rate however, the method has not found general acceptance. 

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