When using engineering materials, the corrosion resistance of materials under corresponding working conditions must be considered.
That is to say, whether the materials will be seriously corroded in this environment, which will lead to the failure of engineering structures.
Therefore, how to evaluate the corrosion morphology and corrosion rate of the material surface under the working conditions is of great practical engineering significance.
To sum up, the evaluation methods of corrosion resistance of engineering materials can be divided into three categories: gravimetric method, surface observation method and electrochemical testing method.
1. Gravimetric method
Gravimetric method is the most basic and reliable quantitative evaluation method in the study of 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 changes in the weight of materials before and after corrosion.
Because of this, it has been widely used in corrosion research, and is the basis for identification and comparison of many modern electrochemical, physical and chemical analysis and evaluation methods.
There are two kinds of gravimetric methods: weight gain method and weight loss method.
Both of them characterize the corrosion rate by the weight difference between the sample before and after corrosion.
The former is to weigh the sample together with all corrosion products after corrosion test, while the latter is to weigh the sample after removing all corrosion products.
When using the gravimetric method to evaluate the corrosion resistance of engineering materials, it should consider whether the corrosion products are easy to fall off during the corrosion process, the thickness and density of corrosion products and other factors, and then decide which method to choose to characterize the corrosion resistance of materials.
The weight-loss method can usually be considered when the corrosion products of materials are loose, easy to fall off and easy to remove.
For example, when evaluating the corrosion resistance of different magnesium alloys through salt spray test, the weight loss method is usually used, as shown in Figure 1.
Fig. 1 Test of corrosion rate of magnesium alloy by weight-loss method
For materials with dense corrosion products, good adhesion and difficult to remove, such as high temperature corrosion of materials, weight gain method can usually be considered, as shown in Fig. 2.
Fig. 2 Weight gain dynamics curve
In order to compare the data of different experiments and different kinds of materials, it is necessary to use the weight change on the potential area as the unit and the average corrosion rate.
According to the density of metal material, it can be converted into the average corrosion depth in unit time.
One of the key operations of weight-loss method is to completely remove corrosion products without damaging the base metal.
When using the weight-loss method to evaluate the corrosion performance of materials, because different researchers will use different sample sizes, corrosion media and test temperatures, the data obtained are difficult to be comparable.
Therefore, in order to solve this problem, people have standardized a standard corrosion test method – salt spray corrosion test.
At present, salt spray test and weight loss test are widely used in industry to characterize the corrosion resistance of materials.
According to the requirements of ASTMB 117, the sample is placed at an angle of 15-30 degrees, and 5% NaCl solution is used for atomization spraying. The test temperature is 35 ℃.
The salt spray experiment requires that the volume of the salt spray box should be large enough, and the salt spray should not be directly sprayed onto the surface of the experiment.
2. Surface observation method
2.1 Macro observation
It refers to the visual analysis of the morphology of materials before and after corrosion and before and after the 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 rough, any detailed corrosion study should be supplemented by this method.
2.2 Microscopic observation
It refers to the metallographic examination or fracture analysis of the corroded sample, or the analysis of the microstructure and phase composition by scanning electron microscope, transmission electron microscope, electron probe, etc., based on which the microscopic corrosion characteristics and corrosion kinetics can be studied.
The micrographs of common corrosion forms in some engineering materials are shown in the figure.
The following points should be noted when observing the corroded samples:
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 local corrosion may not cause significant corrosion on the surface of the material, but develops inside the material.
Pitting corrosion of stainless steel and other materials is an example, as shown in Fig. 3.
Fig. 3 Pitting on stainless steel wire, typical lace can be seen
Second, when observing the cross section morphology of the oxide film of the material, attention should be paid to the use of scanning electron microscope backscattering mode for observation.
When scanning electron microscopy is used to observe corrosion morphology, there are usually two working modes, one is secondary electron phase mode, and the other is backscattering mode.
The secondary electron phase can obtain the surface morphology of the sample by testing the secondary electrons, while the backscattered 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 of the corrosion sample through the backscattering mode, so as to determine whether the oxide film is a single-layer structure or a multi-layer structure, as shown in Fig. 4.
Third, when the material surface is covered with thick corrosion products, we must pay attention to the comprehensive comparison of the morphology before and after the removal of corrosion products when observing the corrosion morphology, so as to obtain accurate conclusions.
The two materials have the same morphology before the removal of corrosion products, and the corrosion morphology may be different after the removal of corrosion products.
For example, the corrosion morphology of 316L stainless steel after being corroded for 4 hours in the mixed solution of Na2SO4 and NaCl at 80 ℃ is basically the same as that of ZE41 magnesium alloy after being corroded for 12 hours in NaCl solution, and the corrosion products are all cracked.
However, after removing the corrosion products, it is found that the corrosion forms of the two are quite different: uniform corrosion occurs in the mixed solution of 316L stainless steel 80 ℃ Na2SO4 and NaCl, as shown in Fig. 5, while pitting corrosion occurs in ZE41, as shown in Fig. 6.
Fig. 5 Corrosion morphology of 316L stainless steel before and after removal of corrosion products after immersion in 80 ℃ Na2SO4 and NaCl mixed solution
Fig. 6 Corrosion morphology of ZE41 magnesium alloy before and after immersion in NaCl solution and removal of corrosion products
3. Electrochemical test method
Electrochemical testing method is a modern research method which can be used to study the corrosion of materials quickly and accurately.
Because the corrosion of materials mostly belongs to electrochemical corrosion, electrochemical testing methods are widely used in corrosion.
Compared with gravimetric method and surface observation method, electrochemical testing method can not only study the corrosion rate of materials, but also deeply study the corrosion mechanism of materials.
After nearly 50 years of development, electrochemical testing methods can be roughly divided into DC testing and AC testing according to the classification of external signals;
According to the system state classification, it can be divided into steady state test and transient test.
DC test includes dynamic potential polarization curve, linear polarization method, cyclic polarization method, cyclic voltammetry method, constant current/constant potential method, etc; AC test includes impedance test and capacitance test.
For steady state test methods, they usually include potentiodynamic polarization curve, linear polarization method, cyclic polarization method, cyclic voltammetry, electrochemical impedance spectroscopy;
Transient test includes constant current/constant potential method, current step/potential step method and electrochemical noise method.
Among many electrochemical testing methods, potentiodynamic polarization curve method and cyclic polarization method are the most basic and commonly used methods.
According to the corrosion electrochemical behavior of materials, materials can be divided into two categories: active dissolved materials and passive materials.
For different kinds of materials, different standards should be adopted when evaluating their corrosion resistance.
For materials with active dissolution behavior (magnesium alloy, carbon steel, low alloy steel, etc.), it is wrong to only use the level of corrosion potential (Ecorr) to evaluate the corrosion performance of materials.
This kind of wrong understanding comes from the fact that we only pay attention to the thermodynamic trend of material corrosion, while ignoring the corrosion dynamic characteristics of materials.
When evaluating the corrosion resistance of active dissolved materials, the primary parameter is the corrosion current (icorr). T
he smaller the corrosion current, the better the corrosion resistance of materials.
This is because the corrosion current is caused by the dissolution of materials.
The polarization curves of AZ91E and MEZ magnesium alloys are shown in Fig. 7.
It can be seen from the figure that:
Although the corrosion potential of MEZ alloy is far lower than that of AZ91E alloy, considering that the corrosion current of MEZ alloy is significantly lower than that of AZ91E alloy, the corrosion resistance of MEZ alloy should be higher than that of AZ91E alloy, which is confirmed by the salt spray corrosion weight loss and metallographic observation results.
Fig. 7 Polarization Curves of AZ91E and MEZ Magnesium Alloys
As long as the corrosion current of the two materials is roughly the same, the corrosion potential is a parameter to be considered.
The higher the corrosion potential, the better the corrosion resistance of the material.
An example can help to better understand this sentence, Fig. 8: When the potential is a, pure magnesium is at the corrosion potential, and pure magnesium is corroded;
However, AZ91D magnesium alloy is in cathode state, and there is no corrosion.
When the potential is b, pure magnesium is at the anode potential and serious corrosion occurs;
In contrast, AZ91D magnesium alloy is still in the cathode state, and no corrosion occurs.
When the potential is c, both pure magnesium and AZ91D magnesium alloy are at the anode potential, but the anode current of AZ91D magnesium alloy is obviously lower than that of pure magnesium, and the corrosion rate of AZ91D is lower than that of pure magnesium.
According to the above three typical cases, the dissolution current of AZ91D alloy is less than that of pure magnesium at each potential, so it can be judged that the corrosion resistance of AZ91D alloy is better than that of pure magnesium.
Based on the above discussion, the evaluation criteria for corrosion resistance of active dissolution materials can be summarized as follows:
First, it depends on the corrosion current. The smaller the corrosion current, the better the corrosion resistance of the material;
When the difference of corrosion current of materials is small, the higher the corrosion potential, the better the corrosion resistance of materials.
For passive materials (aluminum alloy, titanium alloy, stainless steel, nickel alloy, zirconium alloy), when evaluating the corrosion resistance of such materials, the performance of the material’s passive zone should be evaluated, rather than comparing the corrosion current and corrosion potential of materials.
This is because materials can be passivated, so in the process of engineering application, people will use these materials after passivation treatment.
Two parameters characterizing the corrosion properties of materials can be obtained from the potentiodynamic polarization curve: the breakdown potential Eb and the dimensional passive current ipass.
The higher the breakdown potential, the better the corrosion resistance of the material;
The lower the passive current, the better the corrosion resistance.
For example, in 0.1M H3BO3+0.025M Na2B4O7 solution (Fig. 9), compared with as cast pure nickel, nano twin nickel has higher breakdown potential and lower dimensional passivation current. After nano twin, the corrosion resistance of nickel has been significantly improved.
For another example, after the carrier passivation treatment, the breakdown potential of A890 duplex stainless steel has little change, but the passivation current decreases significantly, which indicates that the corrosion resistance of the duplex stainless steel after the carrier wave is significantly enhanced, as shown in Fig. 10.
When evaluating the corrosion resistance of engineering materials, such a very troublesome phenomenon is often encountered, as shown in Fig. 11.
The breakdown potential of 1Cr17Ni2 stainless steel is lower than that of 1Cr12Ni2WMoVNb stainless steel, but the dimensional passive current of 1Cr12Ni2WMoVNb stainless steel is higher than that of 1Cr17Ni2 stainless steel.
According to the evaluation criteria introduced above, it is difficult to judge which material has better corrosion resistance.
Therefore, it is necessary to introduce the third standard for evaluating the corrosion resistance of passive materials, the protection potential Ep.
The protection potential is obtained by testing the cyclic polarization curve, which is used to characterize the self passivation and self repair ability of materials after pitting.
According to the test standard of ASTM cyclic polarization curve, the scanning potential starts from the relative open circuit potential (OCP) – 300 mV to the current density of 1 mA · cm-2, and then starts to scan the negative direction potential until the potential reaches the relative open circuit potential (OCP) – 300 mV, and the scanning speed is 1 mV/s.
The intersection of the negative scanning curve and the anodic polarization curve is the protection potential.
The cyclic polarization curves of 1Cr17Ni2 stainless steel and 1Cr12Ni2WMoVNb stainless steel are shown in Fig. 12.
It can be found that the negative scanning curve of 1Cr17Ni2 stainless steel intersects the anodic polarization curve, while the negative scanning curve of 1Cr12Ni2WMoVNb stainless steel intersects the cathodic polarization curve, which means that 1Cr17Ni2 stainless steel has a protection potential, while 1Cr12Ni2WMoVNb stainless steel does not.
That is to say, 1Cr17Ni2 stainless steel can repair the pitting corrosion hole when the potential drops after the pitting corrosion occurs, so that it can be passivated again;
After the pitting of 1Cr12Ni2WMoVNb stainless steel occurs, the pitting will continue to develop and cannot be repaired.
Combined with the results of cyclic polarization, it can be judged that although the breakdown potential of 1Cr17Ni2 stainless steel is lower than that of 1Cr12Ni2WMoVNb stainless steel, the corrosion resistance of 1Cr17Ni2 stainless steel is better than that of 1Cr12Ni2WMoVNb stainless steel because of the protection potential of 1Cr17Ni2 stainless steel.
To sum up the above discussion, there are three evaluation criteria for evaluating the corrosion resistance of blunt materials:
The higher the breakdown potential, the better the corrosion resistance of the material;
The smaller the passive current is, the better the corrosion resistance is;
The higher the protection potential, the better the corrosion resistance of the material.