Corrosion rate calculations

Corrosion Rate by CBD Somewhat similarly to the Tafel extrapolation method, the corrosion rate is found by intersecting the extrapolation of the linear poi tion of the second cathodic curve with the equihbrium stable corrosion potential. The intersection corrosion current is converted to a corrosion rate (mils penetration per year [mpy], 0.001 in/y) by use of a conversion factor (based upon Faraday s law, the electrochemical equivalent of the metal, its valence and gram atomic weight). For 13 alloys, this conversion factor ranges from 0.42 for nickel to 0.67 for Hastelloy B or C. For a qmck determination, 0.5 is used for most Fe, Cr, Ni, Mo, and Co alloy studies. Generally, the accuracy of the corrosion rate calculation is dependent upon the degree of linearity of the second cathodic curve when it is less than... 

As the corrosion rate, inclusive of local-cell corrosion, of a metal is related to electrode potential, usually by means of the Tafel equation and, of course, Faraday s second law of electrolysis, a necessary precursor to corrosion rate calculation is the assessment of electrode potential distribution on each metal in a system. In the absence of significant concentration variations in the electrolyte, a condition certainly satisfied in most practical sea-water systems, the exact prediction of electrode potential distribution at a given time involves the solution of the Laplace equation for the electrostatic potential (P) in the electrolyte at the position given by the three spatial coordinates (x, y, z). 

A summary of after-test cleaning procedures and corrosion rate calculations are as given below. 

Uncertainty of reference specimen corrosion rate. According to formula of specific uncertainty addition for corrosion rate calculation, its uncertainty has three components ... 

Corrosion rate calculated in milli-inches per year (mpy) is ... 

NORSOK Standard M-506. CO2 corrosion rate calculation model. Oslo Norwegian Technology Standard Institution, 1998. 

NORSOK 2005. M-506 C02 corrosion rate calculation model Standard Norge. 

The exposed uncoated MS and WS panels were drawn from the sites for corrosion rate determination. Corrosion products were removed by pickling solution and the respective corrosion rates calculated by adopting standards [1,2] and the values are given in Table 3.2. 

Corrosion Rate Calculation (from Mass Loss) ... 

ASTM G 31, Practice for Laboratory Immersion Corrosion Testing of Metals—This method describes typical test vessels, metal specimens (coupons), process controls, aeration techniques, coupon placement and supports, exposure times, and corrosion rate calculations. 

It should be realized that the utOity of the tests is limited if the corrosion is not uniform. If there is extensive localized corrosion, the mass loss data will greatly underestimate the maximum corrosion rate, as it will distribute the corrosion over the entire surface of the coupon. Where localized corrosion may be a concern, the investigator may use a micrometer or other type of sensitive instrument to measure the coupon thickness before and after test. Multiple measurements should be made to ensure that the coupon surhice is well-characterized. The thickness loss in mils divided the test duration in years should be compared to the corrosion rates calculated by the above formula. The degree to which the two results agree indicates the degree to which the coupon did indeed undergo uniform corrosion. 

Overlaying of polarization curves of the different materials in a galvanic couple must be done by first converting the current densities into total currents by multiplying the polarization curves by the exposed wetted surface area for each material. The curves then may be overlaid, and if IR drop is not important, the couple current and potential may be predicted. If the coupled potential deviates by 50-100 mV from the open-circuit potential of the anode, then the current may be converted by Faraday s Law into a corrosion rate. If the coupled potential deviates by less than 50-100 mV from the open-circuit potential of the anode, then it is conservative to add the open-circuit corrosion rate of the anode to the corrosion rate calculated from Faraday s Law to get the total anodic corrosion rate. Neither of these methods is exactly accurate, but both will give a reasonable approximation to the true corrosion rate of the anode in the couple. 

Since intergranular corrosion is limited to corrosion at or immediately adjacent to grain boundaries and is detectable at the microstructural level, simple corrosion rate calculations using weight loss measurements provide no information. Standard ASTM tests, such as the oxalic acid etch test... 

The corrosion rates calculated from the formula or taken from the tables will assist in determining how much corrosion allowance should be included in the design based on the expected lifetime of the equipment. 

When the fluid velocity is such that the mass transfer coefficient is of the same order of magnitude as the precipitation rate constant, the corrosion rate cannot be simplified and the entire equation has to be taken into account to perform corrosion rate calculations. 

The corrosion rate calculated from the mass loss of Type 304 SS as a function of temperature and flow rate through the test cell is plotted in Fig. 57. The corrosion rate is found to increase slightly with increasing flow rate at all temperature, possibly because of enhanced mass transfer rates to the metal surface, although the effect is not strong. The important observation from these data is that the corrosion rate is observed to pass through a maximum at about 350°C, as observed by Mitton et al. (Fig. 43) and Guan and Macdonald (Fig. 61) (see below). 

Figure 59. Relationship between the noise resistance as calculated from ENA data and the corrosion rate calculated from weight loss data. Reprinted from Ref 140, Copyright (2009) with permission from Elsevier.

Where the corrosion problem is not general (uniform), and is localized, such as stress corrosion cracking, pitting, crevice, sulfide stress cracking, etc., the material selection should be on the basis of the specific corrosion problem. In these cases the selection procedure is to follow all parts of the above section, except for the corrosion rate calculation and corrosion allowance. 

J Q Corrosion behavior of mico/alumlnum com- posiles in 3.5% NoCI at 25 °C (75 T). (a) Weight-Joss data from simple immersion tests. b) Corrosion rates calculated from the weight-loss data shown in (a). Source Ref 18... 

The electrochemical process of corrosion is complex, and the corrosion rate depends on myriad physical and chemical parameters. Different degrees of corrosive attack are often observed at different locations on the same part seemingly exposed to the same corrosive environment. Figure 7.6 shows typical uniform general corrosion on a steel tube. Most areas show relatively imiform corrosion, while an unusually severe corrosive attack at one location has penetrated through the tube thickness. Multiple corrosion mechanisms can be activated therefore, different failure modes predominate in accelerated and natural tests. This might require the use of different corrosion models in corrosion rate calculation and introduce different confidence levels in... 

CO2 Corrosion Rate Calculation Model, Norsok Standard, M-506, Rev2, June 2005. 

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