A series of papers by Tomashov and coworkers on stainless steel and one by Stern and co-workers on titanium were reviewed and summarised in this journal 1 , 2. Now Greene, Bishop and Stern 3 , in a paper presented to the Detroit meeting of the Electrochemical Society, have reported the extension of the work to chromium, a metal in which considerably greater interest is being taken than was formerly the case. Noble metal additions appear to have two main actions: first, by providing points of low hydrogen overpotential they induce a high anode current density at a high potential over the bulk of the alloy surface, so leading to passivation, and secondly, by somewhat blocking the main part of the surface they reduce the overall anode current density required for passivation.
It may be seen that the corrosion resistance in hot sulphuric and hydrochloric acids is markedly increased by noble metal additions. The authors suggest that noble metals accumulate on the surface during dissolution whether by remaining undissolved or by being re-precipitated , so leading generally to both the effects described above, just as in the previously demonstrated improvement of titanium and stainless steel. In chromium- gold alloy the first effect predominates; in chromium-iridium the second effect.
Chromium, however, exhibits transpassivity, dissolution to Cr VI at very high anode potentials. Consequently its corrosion rate in strongly oxidising nitric acid is increased by noble metal additions, except by those of palladium, osmium and rhodium; palladium and osmium are soluble in nitric acid and thus do not accumulate on the surface, while rhodium, although it is not soluble and accumulates, may be a poor catalyst for the cathodic reduction of nitric acid.
In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript. The corrosion of alloy steels with different amounts of Cr was studied using electrochemical tests, wet—dry cycle corrosion, X-ray diffraction, and Kelvin probe force microscopy. The results show that the content of Cr is positively correlated with the corrosion resistance of bare steel, but the corrosion resistance of atmospheric corrosion does not show the same pattern.
The atmospheric corrosion resistance of Cr-containing steel exhibits three different stages with the change of Cr element content. These three different corrosion rate stages are related to the influence of Cr content on Fe 3 O 4 content in the rust layer. It is well known that the corrosion resistance of materials can be improved by adding corrosion-resistant alloying elements 1 , 2 , 3.
In recent years, a method to improve the corrosion resistance of materials is to form a dense and stable rust layer. One of the typical examples is weathering steel. By adding certain amounts of corrosion-resistant elements, a dense rust layer is formed on the steel surface, which can impede further corrosion process of the substrate to achieve improvement of the steel corrosion resistance 4 , 5 , 6 , 7 , 8.
The recent researches have suggested that addition of Cr, Cu, or P alloying elements can significantly improve the rust layer structure stability of the weathering steel, thereby improving its applicability in severe industrial and marine environments 9 , 10 , 11 , 12 , 13 , 14 , Several studies have focused on the effect of alloying elements on the corrosion resistance. A study by Ma et al. Several studies 17 , 18 , 19 , 20 , 21 found that Cu can enhance the stability of the rust layer when used together with Cr, thus improving the corrosion resistance of weathering steel.
However, a minimum of 0. The rust layer of corrosion-resistant steel after long-term exposure to the atmosphere has a two-layer construction, and a significant difference Cr content presents in the inner and outer layers, respectively Other studies also found that a pseudo-passivation film formed on some Cr-containing steels in the CO 2 corrosion environment, and this film is consisted of Cr OH 3 24 , After analyzing the rust layer structure of Cr-containing steel, Jiang et al.
Yamashita et al. Although all the existing studies so far used several alloying elements together 28 , 29 , it is not difficult to see from these studies that the Cr content and the corrosion resistance have a close correlation. However, the study of the influence of chromium on the initial stages of the atmospheric corrosion of weathering steels by Kamimura and Stratmann 21 found that the positive effect of chromium is not only not apparent in saline environments but also seems to be even harmful by accelerating weathering steels corrosion in coastal environments.
Other researchers 30 , 31 , 32 also pointed out that the presence of 0. Therefore, the effect of the Cr content on the formation of the weather-resistant rust layer in steel, as well as the relationship between the Cr content and the density of the rust layer, has not yet reached a consentient understanding. In this work, the steel samples with eight different Cr contents were studied to identify the relationship between the Cr content and the atmospheric corrosion resistance.
A dry—wet cyclic corrosion test was used to study the effect of the Cr content on the corrosion resistance of the corrosion layer during the atmospheric corrosion process. The purpose of this study is to provide an insight into the influence of the Cr content on atmospheric corrosion resistance. In addition, this study may provide useful data for the development of micro-alloyed corrosion-resistant materials. The metallographic structure of the eight experimental materials is shown in Fig.
It can be seen from the figure that Cr changes the microstructure of the matrix. The Cr content primarily affects the proportion of bainite in the steel. The W1 structure is essentially ferrite with a very small amount of lath bainite.
W2, W3, and W4 are both granular bainite and ferrite. As the Cr content increases, the proportion of granular bainite increases gradually. This occurs because the addition of Cr to a steel can delay the pearlite transformation and promote the bainite transformation, making it easier to form the bainite structure at a slow cooling rate after austenitizing.
To study the atmospheric corrosion behavior of the steels with different Cr contents, dry and wet cycle experiments were carried out on the eight steel samples. Figure 2 shows that, as the corrosion cycles increased, the corrosion rate gradually stabilizes. At the initial stage of the experiment, the main reaction is the corrosion of the steel substrate. Therefore, the corrosion rate gradually decreases with the increase of Cr content. The Cr content showed a certain positive effect on the corrosion resistance in the early stage of corrosion.
However, after the h dry—wet cycle accelerated experiment, the corrosion rate change started to show two different trends for the samples with different Cr contents. The effect of Cr on corrosion rate of the samples with low Cr contents W1—W4 decreased gradually and trended to be the same, while the effect of Cr on corrosion resistance in those with high Cr content W5—W8 still showed a positive correlation with the Cr content.
This is mainly due to the gradual formation of the rust layer on the steel surface during the prolonged experiment time, where the main factor that affects the corrosion resistance has shifted from the corrosion resistance of the steel substrate to the density of the rust layer. After the h dry and wet cycle corrosion experiment Fig.
The dotted line corrosion rate in b is 3. Based on corrosion rate variation, there are two corrosion stages in b , c and three in d. The macroscopic morphology Supplementary Fig. The surface rust layers of W1—W6 are predominantly dark brown, with some yellow-brown rust spots; W7 has a small amount of brick red rust on the surface, and W8 exhibits only a very slight corrosion and the substrate is visible.
There is obvious surface bubbling for W1—W4. The surface rust layers for W5—W7 are relatively flat. W8 is evenly covered with yellowish brown rust and is very dense. The surface of W8, however, is covered by fine and uniform corrosion products, which is very dense with no cracks, thus protecting the substrate. Figure 3 shows the microscopic morphologies of the experimental steels after the removal of the rust.
The surfaces of W1—W4 exhibit many pitting pits and severe uniform corrosion morphology. W5—W7 present many localized corrosion features. There are virtually no obvious corrosion marks on the surface of W8, but a small number of small pits. These results are consistent with the other studies, which had a conclusion that the local corrosion of steel becomes more obvious, and the interface of the base rust layer becomes uneven after adding Cr to steel The scanning electron microscopic SEM observation results of the rust layer cross-section are shown in Fig.
It can be seen from the cross-sectional topography that there is no obvious delamination in the rust layer. For W1—W4, the number of the pitting pits increases gradually as the Cr content increases, the rust layer cross-section is uneven, and there are many microscopic cracks and holes. Samples W5—W7 exhibit significant local corrosion, the bonding between the rust layer and the substrate is weak, and large cracks exist in the rust layer.
The rust layer structure is very dense and exhibits good bonding with the matrix. It can be seen from the EDS line scan that the rust layer near the substrate is enriched in Cr. Cracks and holes also appear in the cross-section of the rust layer.
This is because the rust layer contains various corrosion products, and the densities of different corrosion products are different and smaller than that of the matrix. During the continuous accumulation of corrosion products on the surface of the steel substrate, stress is generated at the interface between the rust layer and the substrate, and corrosion products themselves have poor ductility, resulting in cracks and holes.
The area marked with the red line is general corrosion area and marked by the yellow line is the severe pitting corrosion area. The surface corrosion form changes with Cr content. Localized corrosion of steel becomes more obvious from W2 to W7 samples. The X-ray diffraction XRD analysis on the rust layers of the samples after the wet—dry cycles corrosion test was performed, and the results are shown in Fig. However, the content of each substance is significantly different in the different samples.
Therefore, the K value method was used in order to semi-quantitatively analyze the rust layers to calculate the proportion of different phases in the rust layers.
The results are shown in Table 1. Therefore, for each experimental steel we analyzed the relationship between the corrosion rate and the ratio of Fe 3 O 4 in the rust layer for different corrosion cycles Fig. For each corrosion cycle, the ratio of Fe 3 O 4 in the corrosion product of W8 is consistent with its corrosion rate. This change of Fe 3 O 4 content in the rust layer is consistent with the results from the corrosion rate change curve.
The orange lines in a — d all represent the change curve of corrosion rate, and the black line represents the change of Fe 3 O 4 content of corrosion products.
The polarization curves Supplementary Fig. This is consistent with the traditional understanding of the role of Cr element but not with the results observed in the atmospheric corrosion process. In order to further investigate this difference, the microscopic structure change of the substrate caused by Cr was studied to further clarify the reason for improving the corrosion resistance of the substrate.
Many studies 34 , 35 , 36 investigated the possibility that the steel structure affects the corrosion resistance, and the addition of alloying elements changes the microstructure of the steel matrix, which in turn can affect its corrosion resistance.
Our work showed that, when Cr was added as an alloying element to low-carbon steel, it delayed the pearlite transformation and promoted the bainite transformation, allowing the bainite structure to form more easily after austenitization. It can also be seen from Fig. Therefore, it is considered that the difference of the corrosion resistance in the experiment might be mainly contributed by the structural changes caused by the addition of different amount of Cr.
Supplementary Fig. The structure is composed of ferrite and granular bainite. The locus marked in the figure was where the KPFM testing performed. If passivity is destroyed under conditions that do not permit restoration of the passive film, then stainless steel will corrode much like a carbon or low-alloy steel.
For example, covering a portion of the surface—for example, by biofouling, painting or installing a gasket—produces an oxygen-depleted region under the covered region.
The oxygen-depleted region is anodic relative to the well-aerated boldly exposed surface, possibly resulting in the corrosion of the covered region. Pitting in stainless steel. Under certain circumstances, the passive layer can break down at localized spots on a well-exposed stainless steel surface. When this happens, the metal can corrode in the localized spots. This is called pitting corrosion.
One common cause of pitting corrosion is exposure to aqueous environments that contain chloride. Examples are coastal atmospheres, road salt combined with rainwater, and even tap water containing high levels of chloride. Intergranular corrosion of stainless steel. During the fabrication of stainless steel components or structures, it is possible to degrade the corrosion resistance.
This occurs when austenitic stainless steels e. The grains fall out and the metal loses strength. The increased susceptibility to corrosion by this change in microstructure is called sensitization. Reprinted with permission. Michael Pfeifer, Ph.
He provides metallurgy training and metallurgical engineering consulting to companies involved with product development and manufacturing. He has over 20 years of experience working on failure analysis, root cause analysis, product design, cost reduction, and quality improvement for a wide variety of products and materials.
0コメント