Corrosion acceleration factors

Oxygen corrosion involves many accelerating factors such as the concentration of aggressive anions beneath deposits, intermittent operation, and variable water chemistry. How each factor contributes to attack is often difficult to assess by visual inspection alone. Chemical analysis of corrosion products and deposits is often beneficial, as is more detailed microscopic examination of corrosion products and wasted regions. 

If the cavitation intensity is low and corrosion is a significant accelerating factor, appropriate inhibitors can be useful. Notable successes have been achieved with diesel engine cylinder liners. 

Dealloying is influenced by many factors. In general, any process that increases general corrosion will promote dealloying. However, specific acceleration factors may be further classified into one of three categories metallurgy, environment, and water chemistry. 

Coupled metal (wrought form) Uncoupled corrosion rale (mm/y) Acceleration factor- due to dissimilar metal coupling with Titanium Mild steel ... 

Corrosion rates relate to general corro.sion only and are average rates obtained over about one year s exposure, t Acceleration factors quoted for coupled metal corroding unless indicated by suffix (MS) for mild steel corroding. 

Sharma et al. [153] have devised a gentle accelerated corrosion test using a kinetic rate equation to establish appropriate acceleration factors due to relative humidity and thermal effects. Using an estimate for the thermal activation energy of 0.6 eV and determining the amount of adsorbed water by a BET analysis on Au, Cu and Ni, they obtain an acceleration factor of 154 at 65°C/80% RH with respect to 25 °C/35-40% RH. 

Although such acceleration factors were applied by Fisher et al. [129] in their discussion of the corrosion of Co alloys, there are only a few papers trying to establish the experimental acceleration factors for disk materials. One of these is the work of Novotny et al. [154], These authors observed that carbon-coated CoCr, CoPt, or CoP disks, when exposed to elevated humidity and temperature, show an accumulation of a corrosion product, similar to those seen on uncoated alloys, on the top surface of the carbon. For both overcoated and uncoated disks, oxides and hydroxides of cobalt were detected on the surface, but there was no detectable Cr, Pt, or P. The amount of Co was on the average less on the C-covered disks. The surface Co resulted... 

Examples of their results [154] are shown in the set of curves in Fig. 13. At a given humidity, the Co concentration increases with T the thermal activation energy is about 0.4 eV. At a given temperature, the corrosion increases with an increase in humidity. As the humidity changes from 30 to 90%, the corrosion rate increases about an order of magnitude. The data allow a calculation of the acceleration factors for a variety of conditions. For example, the acceleration factor for 90°C/90% RH with respect to 30°C/40% RH is calculated to be 150. If the product passes a 2-week exposure to 90 °C/90% RH, the test indicates that it will survive in excess of 6 years at 30 °C/40% RH. The values of the acceleration factors, however, may vary from film to film. 

General Guide to Corrosion Rate Acceleration Factors.16... 

In the absence of test data, a useful approximation is as follows for near-equal areas in aqueous environments, the corrosion rate of the anodic member will double. As can be seen in Table 1.1, the acceleration factor varies with the area ratio. 

GENERAL GUIDE TO CORROSION RATE ACCELERATION FACTORS (COUPLED CORROSION RATE/UNCOUPLED CORROSION RATE) IN STATIC SEA WATER1 ... 

Loading frequency and moderate variation of temperature have little effect upon the number of cycles to fracture in dry fatigue. These factors are more important in CF. The effect of frequency is significant because flie corrosion effect is time dependent, and the temperature is known to be an important factor in connection with corrosion. These two factors are mentioned at the same time because they both represent possible ways of accelerating CF tests. Reliable test results can, however, be obtained only if the fatigue acceleration factors are quantitatively known. 

Temperature can also be used as an acceleration factor in a fashion similar to potential. Many materials wiU not pit at a temperature below a critical value that is often extremely sharp and reproducible [56-62]. At low temperatures, extremely high breakdown potentials are observed, corresponding to transpassive dissolution, not localized corrosion. Just above the critical pitting temperature (CPT), pitting corrosion occurs at a potential that is far below the transpassive breakdown potential. This value of CPT is independent of environmental parameters and applied potential over a wide range, and is a measure of the resistance... 

Development of an environmental corrosion intensity factor that facilitates quantification of the acceleration provided by test conditions and enables prediction of performance, based on exposure time, in any combination of field environments. 

Categories of laboratory (accelerated) tests are electro-chemictd, immersion, cabinet, cyclic, and proving grounds. These accelerated tests require (1) selection of environmental and physical conditions representative of service exposure, (2) accelerated corrosion rates without a change in the mechanism, (3) reproducibility of results, and (4) acceleration factors which relate to service performance. 

Accelerated corrosion tests are appealing because the allocated time is often very short. Generally, increased temperature, increased concentration of some of the corrosive species, increased or decreased pH, and applied DC current can all increase the rate of corrosion. However, these acceleration factors can be misused for the sake of saving time, and the test may become meaningless. 

The control of corrosion and erosion of slurry pipelines is covered by Chapter VIII of the ASME Code B31.11. The code correctly points out that in certain cases erosion is an accelerating factor in the internal corrosion of pipelines, by effectively removing scales, oxides, films, and lining. 

Corrosion inhibitor research requires accelerated testing of candidate inhibitors such as REM compounds, before field trial of the most successful candidates. A challenging issue is that an inappropriately designed accelerated inhibitor test could introduce major uncertainties to its results. Acceleration of a corrosion test is usually achieved through the enhancement of the aggressiveness of the test environment in order to intensify major corrosion controlling factors. The identification of major environmental factors that may control the thermodynamics, kinetics and mechanism of a corrosion process is, therefore, the first step in inhibitor test design. 

Cabinet tests. Cabinet testing refers to tests conducted in closed cabinets where the conditions of exposure are controlled and mostly designed to accelerate specific corrosion situations while trying to emulate as closely as possible the corrosion mechanisms at play. Cabinet tests are generally used to determine the corrosion performance of materials intended for use in natural atmospheres. In order to correlate test results with service performance, it is necessary to establish acceleration factors and to verify that the corrosion mechanisms are indeed following the same paths. Modem surface analysis techniques can be quite useful to ascertain that the corrosion products have the same morphologies and crystallographic stmctures as those typically found on equipment used in service. There are basically three types of cabinet tests ... 

Various materials of construction for refinery and petrochemical plant service may exhibit accelerated metal loss under unusual fluid-flow conditions. Attack is caused by a combination of flow velocity (mechanical factors) and corrosion (electrochemical factors) known as erosion-corrosion. 

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