Corrosion resistance of amorphous aluminium-molybdenum alloys in an acidic chloride environment

Pure aluminium corrodes readily in 1 M HCl solution while its supersaturated alloys with Mo, explored in this work, exhibit excellent corrosion properties due to the oxide barrier film formation on the alloy surface. The influence of Mo content in the alloy on the potential and charge distribution across the system alloy |oxide film| solution was studied in situ using dc and ac electrochemical methods. Passivity, a sudden change in electrode impedance, and the onset of secondary passivity and pitting were discussed in terms of the solute segregation, according to the solute-rich mechanism, change in the alumina structure and solid state processes connected with the transport of point defects and spatial distribution of surface charge.     

Corrosion behaviour of aluminium in copper containing environment

Corrosion behaviour of pure aluminium galvanically connected to metallic copper or in the presence of Cu²⁺ ions was investigated by electrochemical measurements in Na₂SO₄ and Na₂SO₄ + NaCl test solutions. It has been found that in aerated Cl⁻ ion containing solutions pitting corrosion of aluminium emerged immediately, while in the absence of oxygen this process was less violent. Effect of passivating pre-treatment of aluminium surface on corrosion behaviour Cu-Al bimetallic system is also demonstrated.     

A newly synthesized glycine derivative to control uniform and pitting corrosion processes of Al induced by SCN anions - Chemical, electrochemical and morphological studies

A newly synthesized glycine derivative (termed GlyD), 2-(4-(dimethylamino)benzylamino)acetic acid hydrochloride, was used to inhibit uniform and pitting corrosion processes of Al in 0.50 M KSCN solutions (pH 6.8) at 25 °C. For uniform corrosion inhibition study, Tafel extrapolation, linear polarization resistance and impedance methods were used, complemented with SEM examinations. An independent method of chemical analysis, namely ICP-AES (inductively coupled plasma atomic emission spectrometry) was also used to test validity of corrosion rate measured by Tafel extrapolation method. GlyD inhibited uniform corrosion, even at low concentrations, reaching a value of inhibition efficiency up to 97% at a concentration of 5 × 10⁻³ M. Results obtained from the different corrosion evaluation techniques were in good agreement. This new synthesized glycine derivative was also used to control pit nucleation and growth on the pitted Al surface based on cyclic polarization, potentiostatic and galvanostatic measurements. The pitting potential (E_pit) and the repassivation potential (E_rp) increased by the addition of GlyD. Thus GlyD suppressed pit nucleation and propagation. Nucleation of pit was found to take place after an incubation time (t_i). The rate of pit nucleation and growth decreased with increase in inhibitor concentration. Morphology of pitting was also studied as a function of the applied anodic potential and solution temperature. Cross-sectional view of pitted surface revealed the formation of large distorted hemispherical and narrow deep pits. GlyD was much better than Gly in controlling uniform and pitting corrosion processes of Al in these solutions.     

Effect of microcrystallization on pitting corrosion of pure aluminium

A microcrystalline aluminium film with grain size of about 400 nm was prepared by magnetron sputtering technique. Its corrosion behaviour was investigated in NaCl containing acidic solution by means of potentiodynamic polarization curves and electrochemical noise (EN). The polarization results indicated that the corrosion potential of the sample shifted towards more positive direction, while its corrosion current density decreased compared with that of pure coarse-grain Al. The EN analysis based on stochastic model demonstrated that there existed two kinds of effect of microcrystallization on the pitting behaviour of pure aluminium: (1) the rate of pit initiation is accelerated, (2) the pit growth process was impeded. This leads to the enhancement of pitting resistance for the microcrystallized aluminium.     

Pitting of zinc: Observations on atmospheric corrosion in tropical countries

A five-nation study has investigated the mechanisms and rates of the atmospheric corrosion of zinc and steel in tropical regions in Australia, Thailand, Indonesia, Vietnam and The Philippines. For the study, 18 exposure sites encompassing severe marine, marine, severe industrial, industrial, marine/industrial, urban and highland environments were established across the countries. At each location, zinc and steel plates were exposed for periods of three months and one year, and measurements were taken of a wide range of surface-response and climatic parameters, including gaseous SO_x and NO_x, airborne salinity, relative humidity (RH) and temperature, rainwater composition, surface temperature and time of wetness (TOW). Exposed plates were used to determine mass loss, the nature of corrosion products (using FTIR and SEM-EDS) and the morphologies of corrosion layers (via SEM-EDS). Regression analysis indicated that the prime factors controlling zinc corrosion rate were climate (temperature and rainfall) and surface-response (TOW), and surprisingly not pollutant levels, despite significant variation in SO_x levels across the sites. SEM studies indicated the presence of pitting below the oxide layers on zinc, particularly those plates exposed at marine and other sites with relatively low SO_x levels. In contrast, no pitting was observed (or pits had very low aspect ratios) in the specimens exposed at sites with high SO_x levels. The possible processes leading to the observed damage patterns are discussed.     

Comparison of susceptibility to pitting corrosion of AA2024-T4, AA7075-T651 and AA7475-T761 aluminium alloys in neutral chloride solutions using electrochemical noise analysis

The susceptibility to pitting corrosion of AA2024-T4, AA7075-T651 and AA7475-T761 aluminium alloys was investigated in aqueous neutral chloride solution for the purpose of comparison using electrochemical noise measurement. The experimentally measured electrochemical noises were analysed based upon the combined stochastic theory and shot-noise theory using the Weibull distribution function. From the occurrence of two linear regions on one Weibull probability plot, it was suggested that there existed two stochastic processes of uniform corrosion and pitting corrosion; pitting corrosion was distinguished from uniform corrosion in terms of the frequency of events in the stochastic analysis. Accordingly, the present analysis method allowed us to investigate pitting corrosion independently. The susceptibility to pitting corrosion was appropriately evaluated by determining pit embryo formation rate in the stochastic analysis. The susceptibility was decreased in the following order: AA2024-T4 (the naturally aged condition), AA7475-T761 (the overaged condition) and AA7075-T651 (the near-peak-aged condition).     

Quantification of pitting in two tempers of 7075 aluminium alloy by non-destructive evaluation

An attempt was made to use the damage parameter, β obtained from non-linear ultrasonic assessment, to quantify pitting damage in 7075 aluminium alloy. β values were found to be different for the two tempers, T6 and T73. β reflected the pitting damage quite well in the initial stages of pitting, during which the non-linearity change in the lattice was mostly governed by the creation of free surfaces due to pitting. However, on prolonged pitting, the β values showed anomalous behaviour. The findings are explained by the operation of counteracting microstructural influences on lattice non-linearity.     

Localized attack of copper and brass in tap water: the effect of Pseudomonas

The presence of Pseudomonas fluorescens in artificial tap water (ATW) affects the composition of the oxide layer and the susceptibility to pitting corrosion of copper and 70/30 brass. The surface layer was investigated by means of a combination of electrochemical and spectroelectrochemical techniques involving cyclic voltammograms, potentiodynamic reduction curves, anodic polarisation curves, weight-loss tests and reflectance spectroscopy.In the sterile conditions the mass loss is lower in brass than in copper while the presence of bacteria enhances the attack in brass. Dezincification in inoculated electrolyte was revealed by microscopic observation, as well as by potentiodynamic reduction curves. Zn dissolution was also supported by spectroscopic evidence.Slow-rate voltamperometric curves were used to determine potential values characteristic of localized corrosion. In the presence of bacteria, the pitting potential moves towards more positive values for both materials but the difference between the repassivation and the pitting potential increases. Bigger and deeper pits can be seen in the presence of microorganisms.     

Corrosion properties of steel protected by nanometre-thick oxide coatings

A comprehensive study of the corrosion properties of low alloy steel protected by 40–50 nm aluminium and tantalum mixed oxide coatings grown by atomic layer deposition is reported. Electrochemical and surface analysis was performed to address the effect of substrate surface finish and whether an oxide mixture or nanolaminate was used. There was no dissolution or breakdown for nanolaminate alumina/tantala stacks in acidic NaCl solution. Localised corrosion (pitting) took place when defects exposing the substrate pre-existed in the coating. Substrate pre-treatment by brushing and H₂–Ar plasma was instrumental to block or slow down pit initiation by reducing the defect dimensions.     

A two step anodizing process of aluminium as a means for improving the chemical and physical properties of oxide films

An anodizing process was developed to form corrosion resistant and hard oxide films on aluminium. The process consists of two steps: first the formation of chromate/phosphate treated layer on the surface of aluminium and secondly anodizing in a sulphuric acid solution. The anodic oxide films formed by the present process contain Cr(III) and phosphate species mostly in the outer part of the porous layer. The films formed by the present process provided a better corrosion resistance to the substrate aluminium from pitting in a chloride medium than the films formed by conventional anodizing and sealed in a boiling chromate solution. Further, Vickers hardness on the cross section of film increased compared with the films formed by conventional anodizing. This two step process can be developed to form other composite oxide films by using different treatments for the first step.     

Localized corrosion of a 2219 aluminium alloy exposed to a 3.5% NaCl solution

This work is focused on the role of intermetallics in pitting corrosion of Al2219 alloy. Second phase particles were characterized by AES, SAM and EDX. Their behaviour in a solution of NaCl was investigated as a function of exposure time. The results confirmed the cathodic nature of the intermetallics with respect to the aluminium matrix. Corrosion products rich in aluminium and oxygen were found to progressively accumulate around the particles and iron was dissolved from the intermetallic, followed by back-deposition. Copper and manganese did not show any major activity. After 32 h of exposure the larger intermetallics were completely covered.     

Effects of sulfur addition on pitting corrosion and machinability behavior of super duplex stainless steel containing rare earth metals: Part 2

To elucidate the effects of sulfur addition on pitting corrosion and machinability behavior of alloys containing rare earth metals, a potentiostatic polarization test, a critical pitting temperature test, a SEM-EDS analysis of inclusions, and a tool life test were conducted. As sulfur content increased, the resistance to pitting corrosion decreased due to the formation of numerous manganese sulfides deteriorating the corrosion resistance and an increase in the preferential interface areas for the initiation of the pitting corrosion. With an increase in sulfur content, the tool life increased due to the lubricating films of manganese sulfides adhering to tool surface.     

Corrosion behaviour of Lotus-type porous high nitrogen nickel-free stainless steels

Corrosion behaviour of three austenitic Lotus-type porous high nitrogen Ni-free stainless steels exposed to an acidic chloride solution has been investigated by electrochemical tests and weight loss measurements. Polarization resistance indicates that the corrosion rate of Lotus-type porous high nitrogen Ni-free stainless steels is an order of magnitude lower than that of Lotus-type porous 316L stainless steel in acidic environment. The localised corrosion resistance of the investigated high nitrogen Ni-free stainless steels, measured as pitting potential, E_b, also resulted to be higher than that of type 316L stainless steel. The influences of porous structure, surface finish and nitrogen addition on the corrosion behaviour were discussed.     

Stress corrosion cracking initiation under the disbonded coating of pipeline steel in near-neutral pH environment

A novel test setup has been used in this study to simulate stress corrosion cracking initiation under a disbonded coating on an X-65 pipeline steel. In this setup, the synergistic effects of cyclic loading, cathodic protection and soil solution environment under disbonded coatings have been considered. When the X-65 pipeline steel was exposed to the test environment, there existed a wide range of corrosion products on the steel surface in the gradient of cathodic protection. Increasing the test time and the maximum stress increased the possibility of stress corrosion cracking initiation in regions with a high susceptibility to pitting corrosion.     

Electrochemical etching of AA5083 aluminium alloy in trihexyl(tetradecyl)phosphonium bis(trifluoromethylsulfonyl)amide ionic liquid

Trihexyl(tetradecyl)phosphonium bis(trifluoromethylsulfonyl)amide ([P_6,6,6,14][NTf₂]) ionic liquid is shown to react with AA5083 aluminium alloy under a two-step anodic polarisation, leading to partial passivation of the surface. Surface characterisation established that an electrochemical etching process had occurred, comparable to acid etching of aluminium. Energy dispersive X-ray spectroscopy (EDXS) and X-ray photoelectron spectroscopy (XPS) results indicated that magnesium de-alloyed from Mg₂Si intermetallic particles and metal fluorides were deposited onto the remaining Mg₂Si sites, leading to a decrease in the anodic corrosion kinetics (to one third of that of the control) as well as a 100 mV vs. SCE increase in the corrosion and pitting potentials.     

Microstructure and corrosion behaviour of pulsed plasma-nitrided AISI H13 tool steel

The effect of pulsed plasma nitriding temperature and time on the pitting corrosion behaviour of AISI H13 tool steel in 0.9% NaCl solutions was investigated by cyclic polarization. The pitting potential (E_pit) was found to be dependent on the composition, microstructure and morphology of the surface layers, whose properties were determined by X-ray diffraction and scanning electron microscopy techniques. The best corrosion protection was observed for samples nitrided at 480 °C and 520 °C. Under such experimental conditions the E_pit-values shifted up to 1.25 V in the positive direction.     

Emeraldine base as corrosion protective layer on aluminium alloy AA5182, effect of the surface microstructure

AA5182 aluminium alloy cold rolled samples were coated by thin films of emeraldine base (EB) obtained from a 5% solution in N-methylpyrrolidinone. Accelerated corrosion tests prove this coating very effective for corrosion protection of aluminium alloys in neutral environment. This study underlines the prominent role of surface cathodic intermetallic particles in pit initiation and coating break down in enhanced corrosion conditions and suggest that, beside the EB barrier properties, the enhanced corrosion resistance observed on the EB coated samples could partly arise from two other mains factors:

•

a weak redox activity of the polymer which passivate the metal,

•

a proton involving self-healing process taking place at the polymer-metal interface, which contributes to delay local acidification in first steps of corrosion on EB coated aluminium surfaces.

     

Influence of reinforcement proportion and matrix composition on pitting corrosion behaviour of cast aluminium matrix composites (A3xx.x/SiCp)

The influence of silicon carbide (SiCp) proportion and matrix composition on four aluminium metal matrix composites (A360/SiC/10p, A360/SiC/20p, A380/SiC/10p, A380/SiC/20p) immersed in 1-3.5 wt% NaCl at 22 °C was investigated by potentiodynamic polarization. The kinetics of the corrosion process was studied on the basis of gravimetric measurements. The nature of corrosion products was analysed by scanning electron microscopy (SEM) and low angle X-ray diffraction (XRD). The corrosion damage in Al/SiCp composites was caused by pitting attack and by nucleation and growth of Al₂O₃ · 3H₂O on the material surface. The main attack nucleation sites were the interface region between the matrix and the reinforcement particles. The corrosion process was influenced more by the concentration of alloy elements in the matrix than by the proportion of SiCp reinforcement and saline concentration.     

Corrosion of carbon steel C1010 in the presence of iron oxidizing bacteria Acidithiobacillus ferrooxidans

In this study, corrosion behaviors of carbon steel C1010 in the presence of an acidophilic, iron-oxidizing bacterial species Acidithiobacillus ferrooxidans were examined. Results showed that A. ferrooxidans cells, with or without attaching to C1010 steel, accelerated its corrosion at a rate of 3–6× those of acidic water, at a pH of 2, without cells. A. ferrooxidans oxidized Fe²⁺ to Fe³⁺ as an energy source and the produced Fe³⁺ rapidly oxidized Fe⁰ to Fe²⁺ was proposed and verified as the reason. In addition, severe pitting corrosion was found on the C1010 steel surface in solutions containing A. ferrooxidans cells.     

Structure–property quantification of corrosion pitting under immersion and salt-spray environments on an extruded AZ61 magnesium alloy

Extruded AZ61 magnesium coupons were exposed to immersion and cyclical salt spray environments over 60 h in order to characterize their corrosion rates. The characteristics of general corrosion, pitting corrosion, and intergranular corrosion were quantified at various intervals. General corrosion was more prevalent on the immersion surface. In addition, more pits formed on the immersion surface due the continuous exposure to water and chloride ions. However, the pits on the salt spray surface showed larger surface areas, larger volumes, and covered more area on the micrographs as compared to the pits on the immersion surface, due to the dried pit debris that trapped chloride ions within the pits.