Electrochemical investigations of pitting corrosion

By using chronopotentiostatic and stepwise potential change experiments with potentiokinetic and galvanostatic testing, the following types of pitting corrosion of stainless steels in chloride-containing solutions have been investigated: sulphate inhibition of pitting; 35%Cr-Fe alloy; Cr-Ni-Mo stainless steels. The circuit resistance was found to be of fundamental importance. The pit passivation potential depends on the intensity of a corrosion attack before potential change only in the case of inhibited solutions. In uninhibited solutions passivation and formation potentials are nearly equal only in the case of potentiostatic circuit conditions. The Cr-Fe alloy and the Cr-Ni-Mo stainless steels show a potential range of repassivating pitting. Of practical interest is the critical potential of stable pitting which decreases with increasing circuit resistance. The beneficial effect of Mo is only valid for the pitting potential obtained potentiostatically and not at higher circuit resistances. Considering the practical meaning of the addition of Mo it may be concluded that this element is probably essentially connected with repassivation of pits and conditioning effects of the passive layer.     

Microbiological corrosion of tanks in long-term storage of gas oil

Abstract

Investigations have been carried out to establish the causes of leakages occurring in long-term gas oil storage tanks. Samples of oil, water, etc. taken from the bottom of the emptied tanks and from blisters and pits found in the tank-bottoms have been examined and shown to contain sulphate-reducing bacteria. All of the evidence obtained points towards corrosion being due to the action of these bacteria rather than to any other corrosive effect.

Recommendations for the cleaning and repair of tanks are given.     

Electrochemical investigations on corrosion protection oils

Tribologically affected surfaces of high‐strength steels often only may be protected against corrosion by the use of specific protecting oils. The protective effectiveness of these products is usually assessed by neutral salt spray test during qualification. As this test method takes a long time and because the reproducibility is poor a new test method was developed based on electrochemistry. It could be shown that a distinction was possible between well and nonprotecting oils based on current density curves and electrochemical impedance spectroscopy.     

Accelerated corrosion test and corrosion failure distribution model of aircraft structural aluminum alloy

Based on corrosion damage data of 1 0 years for a type of aircraft aluminum alloy, the statistical analysis was conducted by Gumbel, Normal and two parameters Weibull distribution function. The results show that aluminum alloy structural member has the corrosion history of pitting corrosion--intergranular corrosion-exfoliation corrosion, and the maximum corrosion depth is in conformity to normal distribution. The accelerated corrosion test was carried out with the complied equivalent airport accelerated environment spectrum. The corrosion damage failure modes of aluminum alloy structural member indicate that the period of validity of the former protective coating is about 2.5 to 3 years, and that of the novel protective coating is about 4.0 to 4.5 years. The corrosion kinetics law of aluminum spar flange was established by fitting corrosion damage test data. The law indicates two apparent corrosion stages of high strength aluminum alloy section material： pitting corrosion and intergranular corrosion/exfoliation corrosion. The test results agree with the statistical fit result of corrosion data collected from corrosion member in service. The fractional error is 5.8% at the same calendar year. The accelerated corrosion test validates the corrosion kinetics law of aircraft aluminum alloy in service.     

The role of different surface damages in corrosion process in alkaline sour media

By applying different electrochemical methodologies to 1018 carbon steel/different electrolytic media interfaces, surfaces with generalised and blistered damages, commonly present in the oil refinery catalytic plants, were obtained. Afterwards, a freshly polished carbon steel surface as well as the damaged surfaces were characterised by electrochemical impedance spectroscopy and scanning electronic microscopy (SEM). The SEM results reveal that the damaged surfaces present films with different physical properties.In spite of the surface modification, the corrosion mechanism of these interfaces in a medium simulating catalytic plants condensates of the refineries in Mexico (0.1 M (NH₄)₂S, 0.4 mM NaCN (10 ppm CN⁻), pH=8.8) turned out to be the same. It was also found that the corrosion process presents the following steps: charge transfer resistance of steel oxidation in the metal/corrosion product film interface and the diffusion processes of Fe²⁺ and H^o ions through the corrosion product film. It was demonstrated that when there was no damage on the surface (freshly polished surface), a homogeneous film was formed instantaneously upon introducing carbon steel into the sour media. The corrosion process in this film occurs in three stages. On the surface with generalised corrosion the three stages are favoured, particularly that of atomic hydrogen process (inducing blistering). Meanwhile, the blistered surface presents a higher charge resistance of steel oxidation and the Fe²⁺ and atomic hydrogen diffusions through the corrosion products are slower than those presented on the other surface states.     

The study of intergranular corrosion in aircraft aluminium alloys using X-ray tomography

Atmospheric corrosion is one of the leading causes of structural damage to aircraft. Of particular importance is pitting and intergranular corrosion, which can develop into fatigue cracks, stress corrosion cracks, or exfoliation. Therefore it is of interest to the Australian Defence Force (ADF) to understand how corrosion ensues in susceptible aircraft aluminium alloys, such as AA2024-T351 and 7050-T7451. However, there are many difficulties in measuring the extent of intergranular corrosion, since it is predominantly hidden below the surface. Traditionally, cross-sectioning has been used to view and measure the depth of attack. In the present work, 2 mm diameter pin specimens were contaminated with a droplet of 3.5% NaCl and exposed to constant humidity that resulted in intergranular corrosion. X-ray computed tomography was then used to non-destructively assess the depth and volume of corrosion both as a function of time in 97% relative humidity, and as a function of relative humidity after 168 h exposure. Both corrosion depth and volume increased with time, but there was evidence for a limiting depth in AA2024. Depth and volume also increased with relative humidity of the environment, for which the time-of-wetness and oxygen concentration of the droplets were considered the important factors in driving the corrosion process.     

Lab and field investigations on localized corrosion of casing

The corrosion of casing was investigated in the laboratory and oilfield by series methods of EIS, SEM, EDS, well logging, and field coupon test. Data reveal the middle vertical profile along the casing is greatly threatened by localized form rather than generalized corrosion. The pit initiates in the micro‐pore of the corrosion scale on the surface of the steel. The development of pitting depends on the water type classified by geochemist. The calcium chloride water shows highest acceleration to pitting due to the two orders of magnitude higher concentration of chloride ion by comparing sodium sulfate and sodium bicarbonate water types. The element analysis and microstructure observation were used to further describe the pit propagation. It is proposed to get a better understanding of the corrosion damage of casing in the petroleum industry.     

Bacterial corrosion of metals. II. Gravimetric investigations of the corrosion of nonferrous metals

The specific kinetic features of the bacterial corrosion of AK-4, AK-6, Д-16л, Д-16п, AMц-8 aluminum alloys; MA-8 magnesium alloy; and БpAЩMц 10-3-1.5 bronze were investigated with the use of tropical strains of bacteria. The protective effectiveness of anodic-oxide coatings applied to aluminum alloys, and that of TsIATIM-221 and AMG-10 compositions were also tested under these conditions.     

Technical note: Staining procedures for characterising biofilms in corrosion investigations

Abstract

Staining techniques with specificity for polysaccharides, anionically substituted polysaccharides, proteins, and polyphenols have been successfully adapted for use on gelatinous films found in certain pitted copper water pipes. The techniques are described and the results of their application both to films in pipes from service and to a range of controls are reported.     

Inhibition of aluminum corrosion in acid solution by environmentally friendly antibacterial corrosion inhibitors: Experimental and theoretical investigations

Protection of Metals and Physical Chemistry of Surfaces - The inhibition performance of penicillin G(I), methicillin(II) and nafcillin(III) on the corrosion of aluminum in a 1 M HCl solution has...     

Direct-to-metal UV-cured hybrid coating for the corrosion protection of aircraft aluminium alloy

Finding eco-efficient and environmentally viable alternatives to chromate coatings represents a fundamental milestone in the aerospace industry. Here, we show a chromate-free approach to protective hybrid coatings on aluminium alloy (AA2024-T3) departing from photoinduced sol–gel and cationic polymerizations. Beginning with a film of n-alkyltrimethoxysilane and diepoxy monomer, we rely on photogenerated superacids to induce the single step formation of two inorganic and organic barrier networks. Such system combines the unique aspects of photopolymerization including fast reactions, temporal control, solvent-free composition and temperature independence. Used without chemical conversion coating or anodizing, some films have passed 2000 h of salt spray testing.     

Microbiological corrosion and measures to protect against such corrosion part VI. Corrosion of copper in the marine atmosphere: A technique

A technique is developed to enable one to study in a sufficiently objective way the processes of the microbiological corrosion of copper and its alloys in the marine atmosphere. This technique is recommended for diagnosing and forecasting corrosion processes in real conditions for the storage and transportation of products. Original Russian Text ? A.A. Gerasimenko, G.V. Matyusha, L.M. Petrova, 2007, published in Korroziya: Materialy, Zashchita, 2007, No. 3, pp. 33–37. Part V. see Corrosion: Materials and Protection, 2005, no. 8, pp. 45–47.     

Accelerated corrosion exposure in ultra thin sheets of 2024 aircraft aluminium alloy for GLARE applications

The effect of corrosion exposure on ultra thin (t < 0.4 mm) 2024-T3 aluminium sheet thickness is investigated. Microstructural analysis showed that for low accelerated corrosion exposure times, no surface deterioration existed and hence corresponding mechanical properties degradation was assumed to be incited by hydrogen diffusion and subsequent embrittlement. Medium exposure times resulted in cross-section reduction due to formation of sub-surface corrosion products, while crevice corrosion was evident for higher exposure times. Tensile specimens were pre-corroded for single or double sides and then mechanically tested. Decrease in mechanical properties was noticed for both L and T sheet rolling directions, even after a few minutes exposure, with transverse rolling direction having higher degradation values for same exposure times. It was noticed that a single sided corroded specimen needs approximately double exposure time to reach the same ductility decrease of a double sided specimen. Quantitative tensile properties degradation results are discussed; the results were correlated to observed corrosion degradation mechanisms. The stepwise ductile-to-brittle transition of tensile fracture mechanism with increasing corrosion exposure time was denoted via analysis of the ‘un-corroded’ region of tensile fracture surface. Single sided specimens exhibited quasi-cleavage fracture surfaces at the region below protected surface to corrosion.     

Modelling aerosol deposition rates on aircraft and implications for pollutant accumulation and corrosion

Abstract

Although corrosion of airframes is a major issue for both commercial and military air fleets, little work has been carried out on how the pollutants that promote corrosion on airframes are deposited (particularly in flight). In this study, a model has been developed that defines the efficiency of aerosol deposition on the leading edges of aircraft as a function of both aerosol size and airspeed. Deposition only occurs with aerosols in the medium size range, with small aerosols remaining in the airstream and large aerosols depositing on but then splashing off an airframe. However, the size range of particles that are deposited decreases with increasing airspeed. Modelling indicates that aerosols in the size range that can readily deposit in flight will remain stable on an airframe and will not be readily blown off. This knowledge of aerosol deposition efficiency enables calculations of total in-flight depositions while factoring in different locations, types of flights and environmental severity. It is estimated that deposition in flight through clouds may be quite significant, as cloud condensation nuclei are in a suitable range for deposition. The relative extents of aerosol deposition during flight and on ground are compared, and it is found that for certain combinations of airfield location and flight scenario, deposition during flight may be comparable to or exceed deposition on the ground.     

Improvement of the experimental setup for investigations on microbially influenced corrosion of stainless steels

A new type of MIC testing facility has been developed based on the data gained through MIC-experiments, and the analysis of the potential shift caused by MIC. A parallel trial experiment was also conducted lasting more than two months. The results demonstrated that the facility enables one to undergo a detailed investigation of responses to MIC behavior. This allows for the collection of a substantial amount of data required for the investigation of MIC. The experiment also demonstrated that the improved facility offers more flexibility to MIC experiments. The facility can maintain predefined conditions allowing experiments to be conducted without outside environmental influence.     

Electrochemical investigations on the corrosion behaviour of galvanized reinforcing steels in concrete with chromate‐reduced cements

The reduction of the chromate contents in cements affects the electrochemical reactions of galvanized reinforcing steels in fresh concrete. By means of electrochemical measurements and bar pull‐out tests the effect of chromates on the formation of protective coatings at the zinc and the bond behaviour have been investigated using chromate‐containing and chromate‐reduced cements. The results have shown that the bond zone was affected as a result of chromate reduction for cement types CEM I and CEM II. The disturbance to the bond zone should be considered as long‐term in relation to the pore structure which is formed. The effects on the bond are largely compensated for as the concrete ages. The results of the related bond strength with displacement paths of 0.1 and 0.5 mm, respectively, are still within the range of expected figures for non‐galvanized reinforcing steel despite the structural disturbance. Under practical aspects it is not necessary that the existing structural disturbance with respect to bond strength is taken into consideration.