GB/T 41756-2022Corrosion of metals and alloys—Method for estimating the atmospheric corrosion resistance of low-alloy steels (English PDF)

3.1 Low-alloy steel

The material that takes iron as the main element, the carbon content is generally below 2%, and the total mass fraction of other alloy elements added is 1.0%~5.0%.

NOTE: Most low alloy weathering steels contain chromium and copper. Some also have added silicon, nickel, phosphorus, or other alloying elements that improve their weather resistance.

3.2 Atmospheric corrosion resistance index; I

The value that characterizes the relative corrosion resistance of a low alloy steel that is calculated from its chemical composition.

4.1 General requirements

Atmospheric corrosion data shall be obtained according to the test in GB/T 14165. The preparation, cleaning and evaluation of specimens shall be in accordance with GB/T 16545.

4.2 Method One -- Prediction method based on short-term corrosion data of steel

4.2.1 This method is mainly a logarithmic curve regression extrapolation method of corrosion loss with time. These atmospheric corrosion data curves generally coincide with the straight line, which can be expressed by formula (1):

Where, C - Corrosion loss, in grams per square meter (g/m2) or microns (µm); A - Corrosion loss when t=1, in grams per square meter (g/m2) or microns (µm); B - Slope that lgC varies with lgt; t - Time, in years (a). Formula (1) can also be written as formula (2):

Where, C - Corrosion loss, in grams per square meter (g/m2) or microns (µm); A - Corrosion loss when t=1, in grams per square meter (g/m2) or microns (µm);

analysis, the expected corrosion loss at any given time can be calculated. See Annex A for calculation examples.

4.2.3 Examples of atmospheric corrosion loss predictions for low-alloy steels in different environments over a 50-year period are given in Annex A.

NOTE: In some circumstances, extrapolating with logarithmic regression can lead to predictions that are slightly lower or slightly higher than actual losses. Especially in very low corrosive environments, the log-log predicted value will be higher than the actual loss. In very corrosive environments, however, the predicted values are lower than the actual losses. For these cases, numerical optimization or composite modeling approaches can provide more accurate predictions.

Nevertheless, the relatively simple logarithmic curve regression extrapolation method mentioned in 4.2.1 is sufficient to provide forecasts for most cases.

4.3 Method Two -- Prediction method based on chemical composition of steel

4.3.1 Two methods for predicting relative corrosion resistance based on composition

The first is based on the Larabee-Coburn data. The Legault-Leckie equation applied in the industrial atmosphere is used to calculate the atmospheric corrosion resistance index (I). It has the advantage of being relatively simple to apply. The disadvantage is that the scope of application is small. It is only applicable to Cu, Ni, Cr, Si and P as alloying elements. The limited range of composition is narrow. The second is based on Townsend data. Its advantage is that there are many alloying elements applicable to steel and a wide composition range. The disadvantage is that the application is relatively complex.

4.3.2 Prediction method based on Larabee-Coburn data

4.3.2.1 The calculation of atmospheric corrosion resistance index (I) is shown in formula (5). The higher the index, the better the corrosion resistance of the steel. Where, % - The mass fraction of chemical composition.

NOTE: For marine and semi-rural climates, similar corrosion resistance indices can be calculated by the Legault-Leckie equation. The order of the calculated atmospheric corrosion resistance index of steels with different components is the same. But because the equation is obtained based on the corrosion data of industrial atmospheric environment, it shall be cautious when applying it to the corrosion resistance evaluation of low alloy steel in other environments.

4.3.2.2 Formula (5) shall be used when the chemical composition of the low alloy steel is within the range of the original test material composition according to the Larabee-Coburn data. The restrictions are as follows:

a) Cu<=0.51%;

b) Ni<=1.10%;

c) Cr<=1.30%;

d) Si<=0.64%;

e) P<=0.12%.

4.3.2.3 Annex B lists out several low-alloy steel corrosion resistance indices calculated by the Larabee-Coburn method (I). Conduct examples of corrosion resistance comparison.

4.3.3 Prediction method based on Townsend data

4.3.3.1 In this method, A and B are calculated according to formula (6) and formula (7).

Where, xi - The mass fraction of each alloying element in steel.

NOTE: A and B are the constants of the corrosion loss exponential function defined by formula (1). a0 and b0 are the coefficients of the three industrial sites given in Table 1. ai and bi are coefficients for each alloying element in the three industrial sites given in Table 1.

After calculating the values of A and B, use formula (2), formula (3) and formula (4) to calculate the corrosion loss, corrosion rate and corrosion time of the three sites.

Since the three site coefficients a0, b0, ai and bi in this method are obtained by fitting the corrosion data obtained in the industrial environment, it shall be cautious when applying it to the corrosion resistance evaluation of low-alloy steel in other environments. […]