Erosion-Corrosion: An Introduction to Flow Induced Macro-Cell Corrosion

By referring the history of the research in each field, cavitation erosion and erosion-corrosion were compared. The start of the research on the cavitation erosion was the generation mechanism of the cavitation impulsive pressure, which brought about the damage to metallic materials. From both sides of impulsive pressure generation mechanism description by theory and measurement of impulsive pressure by the experiment, the generation of cavitation erosion damage has been recognized to be a pure physical or pure mechanical process. As to erosion-corrosion, corrosion must possibly play the leading part in the damage. However, as to the agency of erosion component and its role has not been clarified yet. The shear force of the fluid flow is the most probable agency of erosion. Nevertheless, it is rather irrational to attribute the various forms of erosion-corrosion solely to the shear force of fluid flow over the metal surface.

Damage depth rather than the weight loss of specimen was adopted for representing the extent of cavitation damage to metallic materials. As a result, linear relationship was obtained between the testing duration and the extent of the damage, that is, the damage depth. Furthermore, from this linear relationship a characteristic index was obtained which represented the resistance of material to cavitation attack. In order to represent the extent of damage during the incubation period, where neither weight loss nor damage depth were observed, another parameter named surface increment percentage was introduced. As a result, another characteristic index was obtained, and these indexes coincided with each other not only in physical meaning but also in quantity. Important conclusion drawn from these indexes was that the cavitation erosion mechanism is same in the incubation period as well as in the weight loss period, and that it is also same in laboratory testing apparatuses as well as in actual machines in the field. In the meanwhile, as to the erosion by solid particle impact, the concept of critical impact velocity was introduced to predict the behavior of solid particle at the impact on target material, rolling and skidding. It was made clear that the former causes the damage on the material by plastic deformation and the latter by cutting. This concept was useful to give rationale to the unexpected agreement of the material performance in the field with the result of laboratory test which was conducted under totally different experimental conditions from those in the field: in both cases damage was caused by the plastic deformation process only but without the cutting process at all. The following conclusion was obtained by integrating above research results on cavitation and solid particle impact that the common mechanism generating pure erosion damage in metallic material is plastic deformation.

Pure erosion and pure corrosion were intentionally superimposed and their interaction was observed, expecting that some useful information on the mechanism of erosion-corrosion might be obtained. As a pure erosion process cavitation erosion and slurry erosion were chosen. It has already been made clear in the previous chapter that the damage generation processes of the erosion are both pure mechanical. As a result of experiments, it was found that cavitation erosion was at the initial stage accelerated by corrosion but in the later stage it was inhibited through the adsorption of chloride ions on the metal surface. Alternatively, the erosion always accelerated corrosion by renewing the metal surface. Slurry erosion process, that is, the damage to the metal surface by the impact of solid particle was entirely enhanced by corrosion. Corrosion of the target material was sometimes enhanced and sometimes inhibited depending on the particle impact angle, which was in good accordance with the observation in the previous chapter. The most important discovery in the experiments was that under the superposition of erosion and corrosion the product of the cooperation of erosion and corrosion did not exist. Accordingly erosion-corrosion was judged as a pure electrochemical corrosion process.

Three kinds of impinging jet, namely free jet, submerged jet and jet-in-slit were introduced for producing erosion-corrosion on the specimens of copper and copper alloys. The complete process of impingement attack was reproduced with submerged jet and jet-in-slit but not with free jet: the origin of this sort of erosion-corrosion is the separation of protective oxide layer from the metal surface due to shear force as well as turbulence force; free jet caused only the shear force. When the flow direction of test liquid was reversed in jet-in-slit the turbulence in the flow disappeared, and instead, characteristic flow velocity distribution or fixed vortex was produced on the specimen surface. In accordance with those the localized corrosion with the morphology similar to the differential flow-velocity corrosion or the horseshoe corrosion appeared. The theory of macro-cell corrosion renders a comprehensible rationale to the relationship between the morphology of the localized corrosion and the characteristic flow pattern of liquid as follows: a difference in the flow condition on a metal surface causes the difference in the anodic dissolution rate of the metal, which induces the formation of macro-cell of corrosion. Once a macro-cell is formed the corrosion rate, in particular, the metal dissolution rate at the macro-anode is accelerated through “macro-cell current effect” as well as “surface area ratio effect”. The measurement of macro-cell current was actually carried out during the progress of erosioncorrosion on a jet-in-slit specimen, and a clear difference was recognized in the behavior of anodic polarization curves at the corresponding locations. Thus, it was demonstrated that the erosion-corrosion on the copper alloy is electrochemical, localized corrosion.

Using jet-in-slit apparatus, erosion-corrosion-proof test of 10 kinds of copper base alloys was conducted. Since a 1% copper chloride (II), CuCl₂, aqueous solution was adopted for the test liquid, corrosion rate of the test specimens was accelerated over 200 times higher than the rate of these materials in the field (as used for the potable water valves). Nevertheless, the ranking order of durability for the materials based on the test results agreed well with the ranking based on the experience of engineers who manufactured the water valves. Thus, owing to the materials comparison test, the reliability of jet-in-slit test methodology was confirmed. Further, a jet-in-slit apparatus was improved for conducting tests under the conditions of high temperature and high pressure which simulated the boiler feed water. With this jet-in-slit, corrosion tests were carried out on carbon steel and low alloy steel under various environmental conditions such as temperature, pH, and oxygen concentration as well as fluid flow conditions such as ordinary flow and reverse flow. Based on the test results the generation mechanism of erosion-corrosion on carbon steel in the water at elevated temperatures was clarified, which has made it possible to predict the erosion-corrosion damage on carbon steel under any environmental and fluid flow condition. Lastly, the mutually contradicting intentions of laboratory corrosion testing methodology were discussed: one is to obtain the test result in the shortest testing duration and another is to simulate most closely the corrosion mechanism of materials in the field.

Erosion-corrosion cases on various metallic materials were analyzed. Firstly, the erosion-corrosion damage in the pipeline system of pure copper was taken up, which was set up in the laboratory with the actual tubes and fittings obtained from the industrial market. Thus, the pure copper pipeline in the field was entirely reproduced except for the working liquid: a 1% copper chloride aqueous solution was used as the working liquid which had been proven in Chapter 5 to exert no influence on the corrosion mechanism, but successfully accelerated the corrosion rate. As a result, it was revealed that the erosion-corrosion occurred in the pipeline was not due to the separation of protective film through the shear force or the turbulence, but that it was a differential flow-velocity corrosion. Secondly, the case of differential flow-velocity corrosion in the pump casing of grey cast iron was examined. The cause of macro-cell formation or the localized graphitization corrosion on the pump casing near the shaft-hole was attributed to the characteristic transition of the graphitization corrosion process with the time. Thirdly, three similar cases of wall thinning in carbon steel pipe carrying pure water at elevated temperatures were taken up. The severe wall thinning in the pipe located at the downstream from the orifice or nozzle flow-meter was attributed indirectly to the shift of the electrochemical state of carbon steel surface from active to passive with the rise in the temperature, that is, the active/passive macro-cell corrosion discussed in Chapter 5. Two different types of wall thinning were found: uniform type and localized type. The former was brought about by the active/passive macro-cell due to the difference in oxygen supply, the latter by the similar macro-cell but due to the difference in the wall temperature. Lastly, the cause of the case which appeared to be an accidental or arbitral occurrence of erosioncorrosion in carbon steel pipeline was estimated to be some abnormal operation.

In planning appropriate and effective countermeasures to protect equipment and machines against the attack of erosion-corrosion, it is indispensable to know the true nature of the attack, so that erosion-corrosion is defined as flow induced localized corrosion in the morphology, and as flow induced macro-cell corrosion in the mechanism. The water chemistry controls mainly the chemical property of the environmental liquids such as pH or dissolved oxygen content in order to prevent uniform corrosion, and accordingly this countermeasure is judged rather not suitable for preventing erosion-corrosion which is categorized into localized corrosion. Selection of material depends not only on the corrosiveness of the environmental liquid but also the circumstances of industry. The cathodic protection, which has been long utilized for preventing the electrolytic corrosion of underground buried pipelines, has a high possibility as a countermeasure to mitigate erosion-corrosion damage because it can be applied to localized corrosion. The elimination of those irregularities in fluid flow, which induce the generation of the macro-cell, is recommended most as a successful countermeasure against erosioncorrosion, and the detailed examples are given. Lastly, methods for monitoring the inception as well as the activity of macro-cell are described. Following the concept of “leak before break” a method to prevent the rupture of the pipeline is also shown.