GB/T 19292.2-2018 Corrosion of metals and alloys - Corrosivity of atmospheres - Part 2: Guiding values for the corrosivity categories
1 Scope
This part of GB/T 19292 specifies guiding corrosion values for metals and alloys exposed to natural outdoor atmospheres for exposures greater than one year. This part is intended to be used in conjunction with GB/T 19292.1.
Guiding corrosion values for standard structural materials can be used for engineering calculations. The guiding corrosion values specify the technical content of each of the individual corrosivity categories for these standard metals.
Annex A provides examples for calculating the maximum corrosion damage after extended exposure up to 20 years for six standardized corrosivity categories.
Annex B provides presumed average initial and steady state corrosion rates of standard metals in intervals relative to six standardized corrosivity categories.
Annex C provides the calculation procedure for corrosion damage of steels in regard to their composition.
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 10123 Corrosion of metals and alloys - Basic terms and definitions (GB/T 10123-2001, eqv ISO 8044:1999)
GB/T 19292.1 Corrosion of metals and alloys - Corrosivity of atmospheres - Part 1: Classification, determination and estimation (GB/T 19292.1-2018, ISO 9223: 2012, IDT)
GB/T 19355.1 Zinc coatings - Guidelines and recommendations for the protection against corrosion of iron and steel in structures - Part 1: General principles of design and corrosion resistance (GB/T 19355.1-2016, ISO 14713-1: 2009, MOD)
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3 Terms and definitions
For the purposes of this document, the terms and definitions given in GB/T 10123 and the following apply.
3.1
guiding corrosion value
corrosion rates, mass loss, penetration or other corrosion characteristics expressing the expected corrosive action of the atmospheric environment of a given corrosivity category towards standard metals
3.2
corrosion rate after extended exposure
corrosion rate after exposures longer than one year
3.3
average corrosion rate, rav
yearly corrosion rate calculated as an average value for the first 10 years of atmospheric exposure of a metal
3.4
steady state corrosion rate, rlin
yearly corrosion rate derived from a long-term atmospheric exposure of a metal, not including the initial period
Note: For the purposes of this part, the corrosion rate after 10 years of exposure is considered constant.
4 Principle
The corrosion rate of metals and alloys exposed to natural outdoor atmospheres is not constant with exposure time. For most metals and alloys, it decreases with exposure time because of the accumulation of corrosion products on the surface of the metal exposed. The progress of attack on engineering metals and alloys is usually observed to be linear when the total corrosion mass loss is plotted against exposure time on logarithmic coordinates. This relationship indicates that the total corrosion damage, D, expressed either as mass loss per unit area or corrosion depth, may be given as Equation (1):
(1)
where,
t——the exposure time, a;
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rcorr——the corrosion rate experienced in the first year, g/(m2·a) or μm/a, in accordance with GB/T 19292.1;
b——the metal-environment-specific time exponent, usually less than 1.
5 Prediction of corrosion damage after extended exposure
This procedure may be used in cases where the value corrosion rate in the first year is available or the corrosion damage can be estimated by the procedures in GB/T 19292.1, and the desire is to predict the extent of corrosion after an extended exposure.
The corrosion rate is calculated by substituting the values in Equation (1).
An appropriate metal-environment-specific time exponent (b) value is selected or calculated according to Clause 6. In cases where long-term metal damage data are available, use the b value from this data. In cases where the detailed composition of the metal is not known, select the B1 value from Table 1 for the metal or alloy in question. This is the b value to be used in Equation (1).
The B1 values were taken as the average time exponents from regression analyses of the flat panel long-term results of the ISO CORRAG atmospheric exposure programme.
Foreword i
1 Scope
2 Normative references
3 Terms and definitions
4 Principle
5 Prediction of corrosion damage after extended exposure
6 Extended exposure
7 Criteria for calculation of corrosion rates of structural metals
Annex A (Informative) Example of maximum corrosion damage after extended exposures for corrosivity categories
Annex B (Informative) Average initial corrosion rates in the atmosphere for different corrosion categories and average steady state corrosion rates in intervals relative
Annex C (Informative) Prediction of corrosion damage of steels with regard to steel composition
