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.     

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.     

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.     

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.     

Carbonate stress corrosion of the mains

It is shown that carbonates and bicarbonates are responsible for the major part of events of corrosion cracking in the mains with cathodic protection caused by local damages to protective coatings (and, possibly, for the events of fracture without cathodic protection). Carbonate cracking is initiated in the course of the active-passive transition and is accompanied by a sharply localized reaction with hydrogen, namely, by the decarbonization of the metal in the vicinity of initiated and propagating cracks. We describe the distinctive features of this phenomenon, the methods recommended for its prevention, and the problems, which should be solved to guarantee the safe operation of pipelines. Karpenko Physicomechanical Institute, Ukrainian Academy of Sciences, L'viv; Paton Institute of Electric Welding, Ukrainian Academy of Sciences, Kiev. Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 33, No. 2, pp. 115–122, March–April, 1997.     

Prediction of corrosion fatigue lives of aluminium alloy on the basis of corrosion pit growth law

Tension‐compression and rotating‐bending fatigue tests were carried out using aluminium alloy 2024‐T3, in 3% NaCl solution. The corrosion pit growth characteristics, and also the fatigue crack initiation and propagation behaviour were investigated in detail. The results obtained are summarized as follows: (i) Most of corrosion fatigue life (60–80%) is occupied with a period of corrosion pit growth at low‐stress amplitude. The corrosion pit growth law can be expressed as functions of stress amplitude σ_a and an elapsed time t. (ii) The critical stress intensity factor for crack initiation from the corrosion pit was determined as 0.25 . This value is the same as the threshold stress intensity factor range for crack propagation. (iii) Corrosion fatigue life can be estimated on the basis of corrosion pit growth law and crack propagation law. The estimated fatigue lives agree well with the experimental data.     

Investigation of the interdependence of stress corrosion of austenitic chromium-nickel steels and their susceptibility to intergranular corrosion

The susceptibility of chromium-nickel austenitic steels to intergranular corrosion was studied on stressed and stress-free specimens tested in accordance with GOST [Soviet standard] 6032-58 and in superheated water at high pressures. It was concluded that steels susceptible to intergranular corrosion should not be used in contact with boiler water and chloride solutions.     

Revisiting the effects of molybdenum and tungsten alloying on corrosion behavior of nickel-chromium alloys in aqueous corrosion

Minor alloying additions such as molybdenum (Mo) have major effects on the localized corrosion resistance of corrosion resistant alloys containing chromium. However, progress in alloy development is mostly based upon empirical observations, where any mechanistic insights are largely relegated to the latter stages of localized corrosion (i.e., stabilization and propagation) that are more readily accessible experimentally. For instance, it is well understood that Mo and tungsten (W) affect repassivation of local active, as well as widespread transpassive, corrosion sites and Mo surface enrichment during corrosion is well-documented. In this paper, a comprehensive examination of the functions and mechanism by which selected Mo and W operate to improve the passivity and resistance to breakdown during the initial stages of localized corrosion of the most common Ni-based solid solution alloys is presented. It is shown that Mo and W exert considerable influence on many stages of corrosion, including both passivation and film breakdown, re-enforcing old and introducing more recent ideas in this comprehensive review of the current state of corrosion research on Ni-Cr-(Mo + W) alloys.     

Review of the atmospheric corrosion of magnesium alloys

Mg atmospheric corrosion is induced by a thin surface aqueous layer. Controlling factors are microgalvanic acceleration between different phases, protection by a continuous second phase distribution, protection by corrosion products, and degradation of protective layers by aggressive species such as chloride ions. The Mg atmospheric corrosion rate increases with relative humidity (RH) and concentrations of aggressive species. Temperature increases the corrosion rate unless a protective film causes a decrease. O₂, SO₂ and NO₂ accelerate the atmospheric corrosion rate, whereas the corrosion rate is decreased by CO₂. The traditional gravimetric method can evaluate effectively the corrosion behavior of Mg alloys.