The electrochemistry of corrosion beneath corrosion deposits

Based on the uniform corrosion mechanisms beneath corrosion deposits described in a preceding theoretical study, the present paper shows that certain deposits attain a steady state only at the free corrosion potential. Except for the natural corrosion potential, electrochemical investigation techniques can therefore only be used to study quasi-stationary states, where the electrochemical reactions and transport phenomena are in dynamic equilibrium with the instantaneous thickness of the deposit. The electrochemistry of a metal covered by soluble or anionic insoluble deposits is very close to that on bare metal (deposits transparent to the imposed polarization). Conversely, deposits of the insoluble cationic type compensate nearly integrally the effects of polarization, thus behaving as veritable passive layers. It is also shown that irreversibility effects are present in the growth regime control of deposits under imposed polarization. This may lead to multiple quasi-stationary states. For example, anodic or cathodic pulses can cause an insoluble deposit to change from cationic to anionic, or vice versa. A particular consequence is the existence of a pitting or general anodic depassivation potential for insoluble cationic deposits. Similarly, there is a protection or cathodic passivation potential for insoluble anionic deposits. Altogether, electrochemical methods shall be used, not only to measure corrosion rates, but also to study the intrinsic stability of the feature of observed deposits. This should enable us really to predict long-term corrosion rates.     

Mechanisms of uniform corrosion under corrosion deposits

The liquid-phase transport phenomena which occur at the surface of iron-base alloys during corrosion have been analysed. These mechanisms determine either the maintenance of bare metal or the precipitation of solid corrosion products, the build-up of a corrosion deposit and the control of its thickness, and finally, the kinetics of the electrochemical reactions under the deposit. Although it is shown that pure precipitation-redissolution or direct formation reactions are impossible, the only conceivable mechanisms are nevertheless closely related, because the transport of iron between the metal and the external corrosive medium occurs chiefly either via the solid phase of the deposit (for soluble deposits), or via the liquid phase permeating its porosities (for insoluble deposits). It is also shown that, depending on the precipitation conditions, any given solid compound Fe _n X₂ can lead to three types of deposit with quite different properties. (i) Soluble deposits: moderately protective, steady-state corrosion insensitive to potential, but highly sensitive to turbulence; (ii) Insoluble cationic deposits (controlled by the removal of Fe²⁺ cations by liquid-phase diffusion): highly protective, corrosion rate slightly sensitive to potential, and insensitive to turbulence. (iii) Insoluble anionic deposits (controlled by the diffusional supply of the precipitatable anion X ^n– : slightly or unprotective, corrosion slight or insensitive to the presence of the deposit; possibly profuse deposit if steady state corrosion is not attained. This theoretical analysis can retrospectively explain numerous experimental observations reported in the literature, such as the incubation time before the drop in corrosion rates, the multiple forms of CO₂ and H₂S corrosion, the role of Ca²⁺ ions, erosion-corrosion and bacterial corrosion. This analysis also paves the way for the reliable laboratory prediction of real corrosion rates under deposits.     

Mechanism of local corrosion in corrosion cracking cracks

Translated from Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 26, No. 4, pp. 3–8, July–August, 1990.     

Overcoming intergranular corrosion

Advice is given on the causes of and tests for intergranular corrosion (IGA). This report describes the metallurgist's approach to avoiding the problem and some of the resistant alloys which have been commercialised.     

Top of the REAC tube corrosion induced by under deposit corrosion of ammonium chloride and erosion corrosion

The crystal and deposition behavior of ammonium chloride salt (NH₄Cl) and multiphase flow simulation were investigated by using Aspen software and CFD technology. And the corrosion failure causes of inlet tube explosion of a refinery hydrocracking reactor effluent air cooler (REAC) were studied. The top of 10# carbon steel base tube corrosion is severe, and reveals an inhomogeneous thinning. The field with localized corrosion is mainly distributed in a range of approximately 1.5 m away from the liner tube. The NH₄Cl crystal temperature increases with the increase of feedstock chloride content, and the decrease of injected water volume. The NH₄Cl salt granules mainly distribute in the forepart of an inlet tube of the REAC system. The liquid phase mainly exists in the bottom of an inlet tube, and the gas phase in the top of the tube. Without the enough liquid water, the NH₄Cl in the gas phase crystallizes and deposits on the top of a pipeline, resulting in under deposit corrosion, which interacts with the flow erosive action accelerates the localized corrosion thinning at the top of forepart of an inlet tube. Outside of the range of corrosion failure, the possibility of ammonium salt crystal decreases with decreasing temperature, and the condensed water increases gradually, then the deposited ammonium salts completely dissolve, and reduces the corrosion of downstream system.     

Stress corrosion and corrosion fatigue of metals in high temperature aqueous environments

The dissolved oxygen content of water influences the susceptibility of austenitic stainless steels and reactor pressure vessel steels to environment assisted cracking by raising the electrochemical potential. The relevance of this fact to the integrity of welded stainless steel constructions, deaerator vessels and reactor pressure vessels steels is discussed. In particulal, the use of slow strain rate test procedures involving applied electrochemical potential control is discussed as a means of assessing the susceptibility to cracking of materials in a range of aqueous environments.     

A model for prediction of time from corrosion initiation to corrosion cracking

Prediction of time to corrosion cracking is a key element in evaluating the service life of corroded reinforced concrete (RC) structures. This paper presents a mathematical model that predicts the time from corrosion initiation to corrosion cracking. In the present model a relationship between the steel mass loss and the internal radial pressure caused by the expansion of corrosion products is developed. The concrete around a corroding steel reinforcing bar is modeled as a thick-walled cylinder with a wall thickness equal to the thinnest concrete cover. The concrete ring is assumed to crack when the tensile stresses in the circumferential direction at every part of the ring have reached the tensile strength of concrete. The internal radial pressure at cracking is then determined and related to the steel mass loss. Faraday’s law is then utilized to predict the time from corrosion initiation to corrosion cracking. The model accounts for the time required for corrosion products to fill a porous zone before they start inducing expansive pressure on the concrete surrounding the steel reinforcing bar. The accuracy of the model is demonstrated by comparing the model’s predictions with experimental data published in the literature.     

Metal corrosion for nanofabrication

The annual cost of corrosion has been increasing globally, and it has now reached beyond 3% of the world's gross domestic product. It remains a challenge to reduce or prevent unwanted corrosion effectively after many decades of effort. Nowadays, more efforts are being made to develop anti-corrosion platforms for decreasing the huge cost of corrosion. In parallel, it is also highly expected to be able to use corrosion for producing useful materials with reduced energy consumption. In this review, recent progress in how methods for controlling metal corrosion can be used to produce structure-diversified nanomaterials are summarized along with a presentation of their applications. As a valuable addition to the scientists' toolbox, metal corrosion strategies can be applied to different metals and their alloys for the production of various nanostructured materials; this also provides insights into how metal corrosion can be further prevented and into how corrosion wastage can be reduced.     

The significance of stress corrosion cracking in corrosion fatigue crack growth studies

A model is proposed to account for interactions between fatigue and stress corrosion crack propagation mechanisms in appropriate corrosion fatigue conditions. Tests on an alloy steel, and both wrought and cast aluminium alloys, are reported. Despite the use of very simple coefficients in the equations derived, encouraging results are obtained.     

Understanding environmental impacts on initial atmospheric corrosion based on corrosion monitoring sensors

Atmospheric corrosion monitoring(ACM)sensors were employed to study the initial atmospheric corrosion of carbon steels over a one-month period in six outdoor dynamic atmospheric environments in China.Based on the~250,000 corrosion data sets collected,the environmental impacts of relative humidity,temperature and rainfall on the initial corrosion behavior of carbon steels were investigated.The results showed that rainfall was the strongest environmental factor influencing the initial atmospheric corrosion rate.Relative humidity significantly influenced the corrosion of carbon steels in low-precipitation environments and non-rainfall period.     

Mechanism of corrosion processes at the tip of a corrosion cracking crack

Translated from Fiziko-Khimicheskaya Mekhanika Materialov, No. 5, pp. 60–65, September–October, 1989.     

Control of fretting corrosion

The results of a laboratory study on the effectiveness of coatings and surface treatments in preventing fretting corrosion are presented.     

High-temperature corrosion of superalloys

The performance of gas turbines has been improved by the development of alloys with progressively increasing high-temperature capabilities. While both strength and corrosion resistance are important, the strength requirements have a higher priority, and alloy developments which led to higher strengths also had the effect of reducing the corrosion resistance, particularly with nickel-base alloys. The most important form of corrosion is the accelerated oxidation which takes place when the air or fuel is contaminated with certain impurities, of which alkali metal salts are the most important. This type of attack is generally known as ‘hot corrosion’. Two different forms of hot corrosion have been distinguished. Type I, which is present over a temperature range of about 800–950°C, and type II, which is present over the range 700–800°C. Both processes involve an incubation period, an initiation step, and a propagation stage. Most attention has been given to the propagation stage but, from a technical point of view, the initiation step is the most important process. Mechanisms suggested include the salt fluxing model, the electrochemical model, and the sulphidation–oxidation model. Both the practical and theoretical aspects of the problem will be reviewed.