When engineering materials are used, the corrosion resistance of materials under corresponding working conditions must be considered. That is to say, whether the material will have serious corrosion in this environment, which will lead to the failure of engineering structure. Therefore, how to evaluate the corrosion morphology and corrosion rate of material surface under working conditions is of great practical engineering significance.
In summary, the evaluation methods of corrosion resistance of engineering materials can be divided into three categories: gravimetric method, surface observation method and electrochemical test method.
1 gravimetric method
Gravimetric method is the most basic and effective quantitative evaluation method for corrosion resistance of materials. Although the gravimetric method can not study the corrosion mechanism of materials, it can accurately and reliably characterize the corrosion resistance of materials by measuring the weight changes of materials before and after corrosion. Because of this, it has been widely used in corrosion research, and it is the basis of identification and comparison of many electrochemical, physical and chemical modern analysis and evaluation methods.
The weight method can be divided into weight gain method and weight loss method. Both of them represent the corrosion rate by the weight difference before and after corrosion. The former is to weigh samples together with all corrosion products after corrosion test, while the latter is to weigh samples after removing all corrosion products. When the weight method is used to evaluate the corrosion resistance of engineering materials, the factors such as whether the corrosion products are easy to fall off in the corrosion process, the thickness and compactness of the corrosion products should be considered before deciding which method to characterize the corrosion resistance of the materials. When the corrosion products of materials are loose, easy to fall off and easy to remove, weight loss method can usually be used. For example, the weight loss method is usually used to evaluate the corrosion resistance of different magnesium alloys by salt spray test, Fig. 1.
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Figure 1 corrosion rate of magnesium alloy measured by weight loss method
However, when the corrosion products of materials are dense, with good adhesion and difficult to remove, such as high-temperature corrosion of materials, weight gain method can usually be considered. Fig. 2.
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In order to make the data of different experiments and different kinds of materials can be compared with each other, it is necessary to use the weight change on the potential area as the unit of expression and the average corrosion rate, such as g.m-2h-1. According to the density of metal material, it can be converted into the average corrosion depth per unit time, such as M / A. The conversion formula between these two types of speeds is as follows:
Where a-corrosion rate by weight, G. m-2h-1; b-corrosion rate by depth, mm / A; ρ - density of metal material, g.cm-3.
The corrosion rate V (mm / a) is calculated from the weight change of the sample before and after the corrosion test
Where Δ W - weight loss of sample, G; ρ - density of metal material, G. cm-3; a - sample area, C m2; t - test cycle, H.
One of the key operations of weight loss method is to completely remove the corrosion products without damaging the base metal. The standard methods for removing surface corrosion products of common engineering materials are shown in the table. When using weight loss method to evaluate the corrosion performance of materials, different researchers will use different sample size, corrosion medium and test temperature, so the data obtained are difficult to be comparable. Therefore, in order to solve this problem, a standard corrosion test method salt spray corrosion test is standardized. At present, salt spray test and weight loss test are widely used to characterize the corrosion resistance of materials. According to the requirements of ASTMB117, the sample was placed at 15-30 degree obliquity and atomized by 5% NaCl solution. The test temperature was 35 degrees Celsius. Salt spray test requires that the volume of the salt spray box should be large enough, and it is not allowed to spray salt spray directly on the surface of the experiment.
2. Surface observation method
2.1 macroscopic observation
It is to make naked eye analysis on the morphology of materials before and after corrosion and removal of corrosion products. Attention should also be paid to the morphology and distribution of corrosion products, as well as their thickness, color, density and adhesion; at the same time, attention should be paid to the changes in the corrosion medium, including the color of the solution, the form, color, type and quantity of corrosion products in the solution. Although this observation is very crude, it is supplemented by any fine study of clothing.
2.2 microscopic observation
It is to carry out metallographic examination or fracture analysis on the corroded samples, or analyze the microstructure and phase composition by SEM, TEM, EPMA, etc., so as to study the micro corrosion characteristics and corrosion kinetics. In some engineering materials, the microstructure of common corrosion forms is shown in the figure.
It is important to observe the microscopic corrosion of the specimen
First, when observing the surface morphology, especially some local corrosion morphology, we must pay attention to the observation of corrosion section morphology. This is because the corrosion caused by local corrosion on the surface of the material is not very significant, but develops inside the material. Pitting corrosion of stainless steel and other materials is an example, Fig. 3.
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Second, when observing the cross-section morphology of oxide film, we should pay attention to the back scattering mode of scanning electron microscope. Scanning electron microscopy (SEM) usually has two working modes, one is secondary electron phase mode, the other is backscattering mode. The secondary electron phase can obtain the surface morphology of the sample by measuring the secondary electron, while the backscatter mode can obtain the element distribution on the sample surface by testing the backscattered electrons. The distribution of elements in the oxide film can be easily distinguished by observing the morphology of the oxide film interface by using the backscatter mode, and the oxide film is a single layer structure