Corrosion of 304 stainless steel exposed to nitric acid-chloride environments

In an effort to examine the combined effect of HNO₃, NaCl, and temperature on the general corrosion behavior of 304 stainless steel (SS), electrochemical studies were performed. The corrosion response of 304 SS was bifurcated: materials were either continuously passive following immersion or spontaneously passivated following a period of active dissolution. Active dissolution was autocatalytic, with the corrosion rate increasing exponentially with time and potential. The period of active corrosion terminated following spontaneous passivation, resulting in a corrosion rate decrease of up to five orders of magnitude. The length of the active corrosion period was strongly dependent on the solution volume-to-surface area ratio. This finding, coupled with other results, suggested that spontaneous passivation arises solely from solution chemistry as opposed to changes in surface oxide composition. Increasing NaCl concentrations promoted pitting, active dissolution upon initial immersion, a smaller potential range for passivity, longer active corrosion periods, larger active anodic charge densities preceding spontaneous passivation, and larger corrosion current and peak current densities. In contrast, intermediate HNO₃ concentrations promoted active dissolution, with continuous passivity noted at HNO₃ concentration extremes. During active corrosion, increased HNO₃ concentrations increased the anodic charge density, corrosion current density, and peak current density. The time required for spontaneous passivation was greatest at intermediate HNO₃ concentrations. Susceptibility to pitting was also greatest at intermediate HNO₃ concentrations: the pit initiation and repassivation potentials decreased with increasing HNO₃ concentration until the HNO₃ concentration exceeded a critical concentration beyond which susceptibility to pitting was entirely eliminated. Increasing solution temperature increased the susceptibility to both pitting and active dissolution.     

Corrosion behaviour of a magnesium matrix composite with a silicate plasma electrolytic oxidation coating

The corrosion behaviour of AZ92 magnesium alloy reinforced with various volume fractions of silicon carbide particles (SiCp) and treated by alternating current (AC) plasma electrolytic oxidation (PEO) was investigated in humid and saline environments. For untreated composites, corrosion attack started around the Al-Mn inclusions and gradually developed into general corrosion without significant galvanic coupling between the matrix and the SiCp. PEO coatings consisted mainly of MgO and Mg₂SiO₄, and revealed increased hardness, reduced thickness and slightly higher corrosion resistance with increasing proportion of reinforcement. Pit formation and hydration of the outer layer were the main mechanisms of corrosion of PEO-treated specimens.     

Carbon steel corrosion under anaerobic–aerobic cycling conditions in near-neutral pH saline solutions – Part 1: Long term corrosion behaviour

A corrosion mechanism has been developed to describe tubercle formation along pipeline steels during successive anaerobic–aerobic cycles. Small concentrations of O₂ under nominally anaerobic conditions can lead to the separation of anodes and cathodes. Under subsequent aerobic conditions localized corrosion is then promoted by O₂ reduction on the general magnetite-covered surface. Subsequently, the conversion of magnetite to maghemite passivates the general surface, and focuses corrosion within one major tubercle-covered pit. On switching from aerobic to anaerobic conditions, corrosion is temporarily supported by the galvanic coupling of lepidocrocite (γ-FeOOH) reduction (to γ-Fe-OH·OH) to steel dissolution primarily within the tubercle-covered pit.     

In situ monitoring of the microstructural corrosion mechanisms of zinc–magnesium–aluminium alloys using time lapse microscopy

A novel technique has been developed that enables in situ monitoring of the microstructural wet corrosion mechanisms of zinc–(1–2 wt.% magnesium)–(1–2 wt.% aluminium) galvanising alloys using time lapse optical microscopy. The technique enabled the imaging of the progression of anodic attack, the development of corrosion product rings radially to the anode and pH gradients between anodes and cathodes using an indicator. It was found that corrosion initiated in the binary and ternary eutectic regions within the microstructure of the alloy with preferential de-alloying of MgZn₂ lamellae. After eutectic dissolution, anodic attack proceeded on the primary zinc rich dendrites.     

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.     

A study on the localized corrosion of cobalt in bicarbonate solutions containing halide ions

The localized attack of cobalt in bicarbonate aqueous solutions containing halide ions was investigated using electrochemical techniques, scanning electron microscopy, UV-visible and Raman spectroscopies. Rotating disc and rotating ring-disc electrodes were used to determine the effect of bicarbonate concentration, solution pH, nature and concentration of the halide ions, convection and potential sweep rate on the corrosion processes. These parameters were found to play a key role on the localized attack induced by halide ions by influencing the production of a Co(HCO₃)₂ precipitate on the pit surface. Potentiostatically generated cobalt oxide films (CoO and Co₃O₄) were found to be efficient to reduce pitting corrosion of cobalt.     

The negative difference effect of magnesium and of the AZ91 alloy in chloride and stannate-containing solutions

The negative difference effect of pure magnesium and of the alloy AZ91 was investigated by volumetric tests in NaCl with and without addition of Na₂SnO₃. Hydrogen comes from two sources: H₂ which accompanies localized magnesium dissolution inside the pits and H₂ from H₂O reduction at the passive surface outside the pits. By separating the two parts it could be shown that the rate of hydrogen evolution inside the pits is quantitatively consistent with a two-step EC-mechanism of magnesium dissolution with hydrogen evolution coupled to the second dissolution step. Addition of Na₂SnO₃ does not influence the second step.     

Environmental factors affecting the corrosion behavior of reinforcing steel III. Measurement of pitting corrosion currents of steel in Ca(OH)2 solutions under natural corrosion conditions

Using a simple electrolytic cell, the pitting corrosion current of reinforcing steel is measured in Ca(OH)₂ solutions in presence of chloride and sulfate as aggressive ions. Pitting corrosion current starts to flow after an induction period which depends on the concentration of both the aggressive and the passivating anions. The pitting corrosion current densities reach steady-state values which depend also on the type and concentration of the corrosive and passivating anions. The corrosive action of the aggressive species decreased in the order: SO₄²⁻ > Cl⁻. Corrosion of the steel is found to be governed by a single electron transfer reaction. Raising the temperature decreases the induction period associated with pit initiation and increases the corrosion current associated with pit propagation. From Arrhenius plots, the activation energies for both pit initiation and pit propagation in presence of chloride and sulfate ions are calculated.     

Atmospheric corrosion of field-exposed AZ31 magnesium in a tropical marine environment

Corrosion behavior of AZ31 magnesium in tropical marine atmosphere was investigated. Chloride ions deposition rate played an important role in the corrosion process, which resulted in an obvious fluctuation of the corrosion rate. The corrosion was initiated from pitting corrosion and then evolved into general corrosion as the exposure time extended. Mg₅(CO₃)₄(OH)₂·xH₂O was the dominate products during the whole exposure periods. The products on the specimens weathered for 1, 6 and 12 months slightly suppressed the corrosion process, while that generated after 24 months of exposure exhibited good protective ability against further corrosion attacks.     

Pitting corrosion behaviour of austenitic stainless steels - combining effects of Mn and Mo additions

Mn and Mo were introduced in AISI 304 and 316 stainless steel composition to modify their pitting corrosion resistance in chloride-containing media. Corrosion behaviour was investigated using gravimetric tests in 6 wt.% FeCl₃, as well as potentiodynamic and potentiostatic polarization measurements in 3.5 wt.% NaCl. Additionally, the mechanism of the corrosion attack developed on the material surface was analysed by scanning electron microscopy (SEM), X-ray mapping and energy dispersive X-ray (EDX) analysis. The beneficial effect of Mo additions was assigned to Mo⁶⁺ presence within the passive film, rendering it more stable against breakdown caused by attack of aggressive Cl⁻ ions, and to the formation of Mo insoluble compounds in the aggressive pit environment facilitating the pit repassivation. Conversely, Mn additions exerted an opposite effect, mainly due to the presence of MnS inclusions which acted as pitting initiators.     

Effect of heat treatment on corrosion behaviour of magnesium alloy AZ91D in simulated body fluid

The research explored ways of improving corrosion behaviour of AZ91D magnesium alloy through heat treatment for degradable biocompatible implant application. Corrosion resistance of heat-treated samples is studied in simulated body fluid at 37 °C using immersion and electrochemical testing. Heat treatment significantly affected microgalvanic corrosion behaviour between cathodic β-Mg₁₇Al₁₂ phase and anodic α-Mg matrix. In T4 microstructure, dissolution of the β-Mg₁₇Al₁₂ phase decreased the cathode-to-anode area ratio, leading to accelerated corrosion of α-Mg matrix. Fine β-Mg₁₇Al₁₂ precipitates in T6 microstructure facilitated intergranular corrosion and pitting, but the rate of corrosion was less than those of as-cast and T4 microstructures.     

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.     

Real time pit initiation studies on stainless steels: The effect of sulphide inclusions

It has long been accepted that manganese sulphide favours pitting on stainless steels. However, there are different standpoints on the most important mechanism for pit initiation; due to dissolution of sulphide inclusions, chromium depletion around the inclusion or mechanical rupture of the passive film by metal chlorides. Analysing the pitting potential and metastable pitting rates on different grades of stainless steels has rationalised the effect of sulphide content on pitting corrosion resistance. In situ atomic force microscopy (AFM) has been used in conjunction with conventional electrochemical techniques for imaging real time pit initiation events.     

Comparison of corrosion pitting under immersion and salt-spray environments on an as-cast AE44 magnesium alloy

The corrosion mechanisms of pitting, intergranular corrosion, and general corrosion were examined on an AE44 magnesium alloy subjected to immersion and salt-spray environments. The two environments show similar trends with respect to weight loss and thickness loss, although the immersion environment induces greater amounts of weight loss of magnesium. With respect to the corrosion mechanisms, the two environments show definitive trends, owing to the continuous presence of water in the immersion environment allowing more and larger pits to form as compared to the salt-spray environment. The immersion environment was more deleterious than the salt-spray environment for magnesium.     

Simulation study on function mechanism of some precipitates in localized corrosion of Al alloys

The function mechanism of different types of aging precipitates in localized corrosion of Al alloys was studied. The function mechanism of the precipitates of θ (Al₂Cu) and η (MgZn₂) is validated. The precipitate of θ containing noble element Cu is cathodic to the alloy base, resulting in the anodic dissolution and corrosion of the alloy base at its adjacent periphery. The precipitate of η containing active element Mg is anodic to the alloy base, anodic dissolution and corrosion occur on its surface. Meanwhile, a localized corrosion mechanism conversion associated with the precipitate of T1 (Al₂CuLi) is advanced, which contains noble element Cu and active element Li simultaneously. The precipitate of T1 is anodic to the alloy base and corrosion occurs on its surface at the beginning. However, during its corrosion process, the preferential dissolution of Li and the enrichment of noble element Cu make its potential move to a positive direction. As a result, the corroded T1 precipitate becomes cathodic to the alloy base at a later stage, leading to the anodic dissolution and corrosion of the alloy base at its adjacent periphery.     

Corrosion stress relaxation and tensile strength effects in an extruded AZ31 magnesium alloy

The corrosion effects on the tensile and stress relaxation behavior of an extruded AZ31 magnesium alloy subjected to immersion and salt-spray environments have been investigated. Specimens were simultaneously corroded and stress relaxed in a 3.5 wt.% NaCl solution and then put under a tensile test to failure to determine the stress–strain response over a 60 h test matrix. The AZ31 magnesium alloy shows an evident relaxation in 3.5 wt.% NaCl at room temperature. According to optical and scanning electron microscopy investigations, the fracture surfaces for the immersion environment show a high sensitivity to stress corrosion cracking.     

Passivity of polycrystalline NiMnGa alloys for magnetic shape memory applications

The corrosion and passivation behaviour of bulk polycrystalline martensite Ni₅₀Mn₃₀Ga₂₀ and austenite Ni₄₈Mn₃₀Ga₂₂ alloys was compared in electrolytes with different pH values. Linear anodic and cyclic potentiodynamic polarisation methods and anodic current transient measurements have been conducted for the alloys and their constituents to analyze free corrosion, anodic dissolution and passive layer formation processes. Electrochemically treated alloy surfaces were characterized with scanning electron microscopy (SEM) and angle-resolved x-ray photoelectron spectroscopy (XPS). The electrochemical response of both alloys is in principal similar and is dominated by the Ni oxidation. In acidic solutions (pH 0.5 and 5) a slightly higher reactivity is detectable for the martensitic alloy which is mainly attributed to enhanced dissolution processes at the multiple twin boundaries. In weakly acidic to strongly alkaline solutions (pH 5-11) both alloys exhibit a low corrosion rate and a stable anodic passivity. While air-formed films comprise NiOOH, Ga₂O₃ and MnO₂, passive films formed in near neutral media (pH 5-8.4) are composed of Ni(OH)₂, NiOOH and Ga₂O₃ in the outer region and of NiO, MnO₂ and MnO in the metal-near region.     

Investigation of AISI 1040 steel corrosion in H2S solution containing chloride ions by digital image processing coupled with electrochemical techniques

This paper presents a study of AISI 1040 steel corrosion in aqueous electrolyte of acetic acid buffer containing 3.1 and 31 × 10⁻³ mol dm⁻³ of Na₂S in both the presence and absence of 3.5 wt.% NaCl. This investigation of steel corrosion was carried out using potential polarization, and open-circuit and in situ optical microscopy. The morphological analysis and classification of types of surface corrosion damage by digital image processing reveals grain boundary corrosion and shows a non-uniform sulfide film growth, which occurs preferentially over pearlitic grains through successive formation and dissolution of the film.     

Use of inkjet printing to deposit magnesium chloride salt patterns for investigation of atmospheric corrosion of 304 stainless steel

Inkjet printing was used to deposit MgCl₂ salt patterns on 304 stainless steel foils to investigate atmospheric corrosion. Results were found to be more consistent if initial hydration (1 h at ∼90% RH) of the printed salt pattern was carried out. The pit diameter following exposure at 45% RH and 300 K for 24 h was found to increase with the diameter of the original salt deposit, which is consistent with the idea of cathodic limitation of the pit current. For a constant deposition area, the pit diameter increases with increased salt deposition density, which may be associated with a lower ohmic drop resulting from a higher droplet, or could be influenced by enhanced corrosion during the initial hydration stage.     

Localised dissolution of iron in buffered and non-buffered chloride containing solutions

Pitting corrosion of pure iron was studied by using conventional samples, and artificial pit electrodes. Experiments were conducted in solutions of 0.01, 0.1 and 1 mol dm⁻³ NaCl at pH values of 7, 10, 11 and 12; and in borate buffer solutions with the same chloride concentrations and pH 8.7 and 9.2. Four times higher concentration of borate salt was required to reach inhibiting capacity of the hydroxyl anions, as determined via pitting potentials. From measurements of solution resistance, the increase in local conductivity due to dissolved corrosion products exuded from the pit was calculated for each bulk solutions. For the artificial pit electrodes, contribution of the borate species to the internal pit solution conductivity in low chloride solution was associated with the difference between transition potential, E_T, values for buffered and non-buffered solutions, and this contribution was also used to explain the non-linear dependence of E_T on [Cl⁻]. Further analysis was conducted using the concept of the critical i.x parameter for stable pit propagation.