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GB/T 41756-2022: Corrosion of metals and alloys -- Method for estimating the atmospheric corrosion resistance of low-alloy steels
GB/T 41756-2022
GB
NATIONAL STANDARD OF THE
PEOPLE’S REPUBLIC OF CHINA
ICS 77.060
CCS H 25
Corrosion of metals and alloys - Method for estimating the
atmospheric corrosion resistance of low-alloy steels
ISSUED ON: OCTOBER 12, 2022
IMPLEMENTED ON: FEBRUARY 01, 2023
Issued by: State Administration for Market Regulation;
Standardization Administration of the People's Republic of China.
Table of Contents
Foreword ... 3
1 Scope ... 4
2 Normative references ... 4
3 Terms and definitions ... 4
4 Program ... 5
5 Report ... 14
Annex A (informative) Method One -- Examples of prediction method based on short-
term corrosion data on steel ... 15
Annex B (informative) Method Two -- Examples of prediction method based on
chemical composition of steel ... 20
Annex C (informative) Method Three -- Examples of prediction methods based on
environmental factors and steel chemical composition ... 21
Corrosion of metals and alloys - Method for estimating the
atmospheric corrosion resistance of low-alloy steels
1 Scope
This document presents three methods for evaluating the atmospheric corrosion
resistance of low-alloy steels. Method One is the prediction method based on short-term
corrosion data of steel. Method Two is the prediction method based on the chemical
composition of steel. Method Three is the prediction method based on environmental
factors and steel chemical composition.
Method One in this document applies to the use of existing short-term atmospheric
corrosion data in different environments for specific grades of low alloy steel. Evaluate
its long-term corrosion damage. Method Two is suitable for evaluating the relative
corrosion resistance of low alloy steel based on the atmospheric corrosion resistance
index (I). Method Three is suitable for estimating the corrosion loss of low alloy steel
at different times in different environments.
NOTE: These three methods are calculated based on the experimental data of flat bare steel exposed
specimens. The actual atmospheric corrosion rate is much higher when the low alloy steel is kept
wet for a long time or is severely attacked by some salts or other corrosive chemicals.
2 Normative references
The following referenced documents are indispensable for the application of this
document. For dated references, only the edition cited applies. For undated references,
the latest edition of the referenced document (including any amendments) applies.
GB/T 14165, Corrosion of metals and alloys - Atmospheric corrosion testing -
General requirements for field tests
GB/T 16545, Corrosion of metals and alloys - Removal of corrosion products from
corrosion test specimens
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
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 Program
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 fun...