Talking about the evaluation of corrosion

Now let's talk about the evaluation of uniform corrosion and local corrosion, but this evaluation is very complicated and difficult to be precise.

Evaluation of uniform corrosion

The degree of uniform corrosion or general corrosion is generally expressed by the average corrosion rate. The average corrosion rate has two expression methods: corrosion rate and erosion rate. The corrosion rate is the amount of corrosion per unit area per unit time; the depth of penetration per unit time is called the erosion rate. The corrosion rate is very convenient for the comparison of the same metal, but for different metals, although the corrosion rate is the same, the corrosion rate of light metals aluminum and magnesium is much larger. Therefore, the comparison of dissimilar metals is better in terms of erosion rate. There are many ways to express the corrosion rate due to the different units used. The conversion of corrosion rate and erosion rate can be performed according to the following table.

Corrosion rate by quality

Depending on whether the corrosion product is easy to remove or firmly attached to the test piece, it can be expressed by weight loss or weight gain per unit area per unit time. The most commonly used method is weightlessness:

Kw=â– 

In the formula: Kw-corrosion rate, g /(m2·h); S-specimen surface area, m2; t-test time; W0-pre-test specimen quality; W-test specimen quality after test.

In addition to gram/(m2·hour) [g /(m2·h)], the corrosion unit is also commonly expressed in grams/(cm2·hour) [g /(cm2·h)]. Milligram/(decimeter 2·day) [mg/(dm2·d)] is commonly used abroad, and the code is mdd. See the table for the conversion of various unit corrosion rates.

Corrosion rate expressed by corrosion depth and thickness change

It is often called the annual corrosion depth, and the common unit is millimeter/year (mm/a). It is also commonly used abroad as mil/year (mil/y) code mpy or inch/year (in/y) code ipy (1 mil=10- 1 inch = 0.0254 mm). Sometimes micron/year (μm/a) is also used to express the erosion rate.

According to the density D of the metal, the corrosion rate expressed by the weight change can be converted into the corrosion depth:

Kd（mm）=■〔 g /（m2·h）〕

Converted from corrosion current to corrosion rate

After the value of the corrosion current is measured by the electrochemical method, it can be converted into the corrosion rate according to Faraday's law: [g /(cm2·h)]

Kw=■〔 g /(m2·h)〕

In the formula: Kw-corrosion rate, g /(cm2·h); Ic-corrosion current, A/cm2; M-metal gram atom weight; n-metal ion valence; F-Faraday constant, 26.8 A·h.

Assessment of local corrosion

The evaluation of local corrosion degree is more complicated, and there is no unified quantitative evaluation standard. There are many forms of local corrosion of metals, and the changes in physical and mechanical properties are also different. For example, pinhole corrosion only reflects changes in the corrosion depth at the pinholes, while other parts have no obvious changes; another example is intergranular corrosion. Although the quality and dimensions of the metal have not changed much, its mechanical properties have dropped significantly. Therefore, the evaluation of local corrosion cannot be evaluated by simple quality changes or changes in external dimensions. It is necessary to use appropriate physical and mechanical property change indicators to evaluate according to the form of corrosion. At present, the evaluation of pitting corrosion adopts comprehensive evaluation of pitting density, average pitting depth, and maximum pitting depth. Intergranular corrosion and stress corrosion are evaluated by the loss of strength performance before and after corrosion. The loss of mechanical strength can be expressed by the following formula:

Ke=■×100%

In the formula: Ke- the rate of decrease in tensile strength after corrosion, %; Rb0- the compressive strength before corrosion; Rm- the tensile strength after corrosion.