Monitoring corrosion rates and localised corrosion in low conductivity water

Monitoring of low corrosion rates and localised corrosion in a media with low conductivity is a challenge. In municipal district heating, quality control may be improved by implementing on-line corrosion monitoring if a suitable technique can be identified to measure both uniform and localised corrosion. Electrochemical techniques (LPR, EIS, crevice corrosion current) as well as direct measurement techniques (high-sensitive electrical resistance, weight loss) have been applied in operating plants. Changes in the corrosion processes are best monitored in non-aggressive, low conductivity media with sensitive electrical resistance technique and crevice corrosion current measurements.     

Comparative evaluation of corrosion behaviour of type K thin film thermocouple and its bulk counterpart

This paper investigates the corrosion behaviour of type K thermoelements and their thin films, and compares the performance of chromel–alumel thin film thermocouple with its wire counterpart before and after exposure to 5% NaCl medium. Potentiodynamic polarisation tests reveal that chromel and alumel films are more “noble” than their respective wires. Alumel corrodes faster when coupled with chromel in films than as wires. Secondary electron micrographs and electrochemical impedance spectroscopy measurements suggest that chromel shows localised corrosion while alumel undergoes uniform corrosion. Corrosion adversely affects the thermocouple output and introduces an uncertainty in the measurement.     

The influence of pre-straining on the corrosion fatigue performance of two hot-dip galvanised steels

The corrosion fatigue behaviour of a hot-dip galvanised ferritic pearlitic steel and of a hot-dip galvanised Ti-alloyed high-strength interstitial free steel was investigated. The testing mode was constant displacement, fully reversed plane bending. Both materials were tested as received and 10% uniaxially pre-strained. The effect of pre-straining on the zinc coating was investigated using polarisation resistance measurements and the scanning reference electrode technique (SRET). It was found that pre-straining is detrimental to the corrosion fatigue behaviour of both steels, due to damage to the zinc coating, leading to increased localised corrosion and in general higher corrosion rates.     

Corrosion properties of active screen plasma nitrided 316 austenitic stainless steel

AISI 316 austenitic stainless steel has been plasma nitrided using the active screen plasma nitriding (ASPN) technique. Corrosion properties of the untreated and AS plasma nitrided 316 steel have been evaluated using various techniques, including qualitative evaluation after etching in 50%HCl + 25%HNO₃ + 25%H₂O, weight loss measurement after immersion in 10% HCl, and anodic polarisation tests in 3.5% NaCl solution. The results showed that the untreated 316 stainless steel suffered severe localised pitting and crevice corrosion under the testing conditions. AS plasma nitriding at low temperature (420 °C) produced a single phase nitrided layer of nitrogen expanded austenite (S-phase), which considerably improved the corrosion properties of the 316 austenitic stainless steel. In contrast, AS plasma nitriding at a high temperature (500 °C) resulted in chromium nitride precipitation so that the bulk of the nitrided case had very poor corrosion resistance. However, a thin deposition layer on top of the nitrided case, which seems to be unique to AS plasma nitriding, could have alleviated the corrosion attack of the higher temperature nitrided 316 steel.     

An innovative specimen configuration for the study of Mg corrosion

Based on an analysis of galvanic corrosion research, the research reported herein was formulated to examine the measurement of polarisation curves for Mg to develop a methodology whereby reliable polarisation curves can be measured for Mg. Cathodic polarisation curves were measured for high purity Mg in 3.5% NaCl saturated with Mg(OH)₂ using three specimen types: (i) mounted specimens, (ii) specimens hung by fishing line and (iii) plug-in specimens. Cathodic polarisation curves were evaluated to yield the corrosion current density i_corr and the corresponding corrosion rate P_i, which was compared with the corrosion rate evaluated from hydrogen evolution measurements, P_H, and the corrosion rate evaluated by weight loss measurements, P_W. Mounted specimens produced values of corrosion rate, P_i, three times larger than values of corrosion rate, P_i, for plug-in specimens, attributed to crevice corrosion in the mounted specimens. Crevice corrosion in Mg is totally unexpected from prior research. The plug-in specimen configuration was designed to have no crevice and to allow simultaneous measurement of P_H and P_i; P_i was consistently less than P_H and indicated an apparent valence for Mg of 1.45 in support of the Mg corrosion mechanism involving the uni-positive Mg⁺ ion. The plug-in specimen has advantages for the study of Mg corrosion.     

Organic molecules showing the characteristics of localised corrosion aggravation and inhibition

The behaviour of imidazoline and an acid functionalised resorcinarene as steel corrosion inhibitors in carbon dioxide (CO₂)-saturated brine solutions has been studied using an electrochemically integrated multi-electrode array namely the wire beam electrode (WBE). Both imidazoline and resorcinarene acid provided excellent inhibition to general CO₂ corrosion; however imidazoline was found to aggravate localised corrosion by creating a small number of major anodes that focused on a small area of the WBE surface, leading to highly concentrated anodic dissolution. The resorcinarene acid showed distinctively different behaviour by generating a large number of minor anodes randomly distributing over the WBE surface, leading to insignificant general anodic dissolution. These results indicate that resorcinarene acid provided effective localised corrosion inhibition by promoting a random distribution of insignificant anodic currents.     

Morphology of corroded surfaces: Contribution of cellular automaton modelling

A simple model describes uniform and localised corrosion. We use a mesoscopic cellular automata (CA) type approach, which has proved a powerful tool in the study of rough metal–electrolyte interfaces. The model accounts for two corrosion kinetics and their relative spatial localisation. The competition between the two forms of kinetics of corrosion is shown to reproduce several types of morphology of corrosion ranging from narrow pits or larger cavities to rough surfaces. Simulation and experimental results are compared providing insight on the relation between kinetics and morphologies of the corrosion front. We emphasize the importance of stochastic simulations in relation to CA.     

Corrosion behaviour of low temperature plasma carburised 316L stainless steel in chloride containing solutions

The electrochemical corrosion behaviour of the carburised (expanded austenite) layer on 316L austenitic stainless steel produced by low temperature plasma carburising has been studied in 0.5 M NaCl and 0.5 M HCl + 0.5 M NaCl solutions. The present work focuses on the variation of the corrosion behaviour of the carburised layer with depth from the surface and the effect of carbon concentration on electrochemical behaviour. The results show that the carburised layer has excellent resistance to localised corrosion. There exists a critical carbon concentration, above which the expanded austenite possesses excellent resistance to both metastable pit formation and pit growth.     

The corrosion of two aluminium sacrificial anode alloys in SRB-containing sea mud

Corrosion of two sacrificial anodes in marine sediment with and without sulphate-reducing bacteria (SRB) was performed. Weight loss experiments indicated that the corrosion rate of Al–Zn–In–Sn was 2–3 times higher than that of Al–Zn–In–Mg–Ti in the SRB-containing sediment. Electrochemical analysis suggested that the corrosion rates of the two anodes were enhanced substantially by SRB. Surface analysis revealed the localised corrosion of the two sacrificial anodes in abiotic and biotic sediments. The concentration of Al³⁺ in the surface of the samples immersed with SRB was lower than that of the samples without SRB.     

Improving the corrosion behaviour of powder metallurgical 316L alloy by prepassivation in 20% nitric acid

The electrochemical improvement of PM 316L stainless steels by pre-alloyed powder prepassivation in 20% nitric acid in comparison with as-received specimens has been studied in this work. For comparison purposes a simultaneous study was carried out on similar composition wrought AISI 316L steels. Corrosion resistance was studied using evolution of the corrosion potential vs. time, anodic polarisations curves and Zero Resistance Ammeter technique. Reductions of the corrosion rate (i_corr) were observed in prepassivated specimens in neutral chloride media. Crevice resistance was higher for prepassivated specimens and for higher densities and annealing as post-heat treatments.     

Mechanism of corrosion of cast iron covered by aged corrosion products: Application of electrochemical impedance spectrometry

This work focuses on the corrosion of cast iron in a drinking water network. The study is performed in water and in a sandy soil saturated with the same water. In the presence of corrosion products, the inner wall surface shows a lower corrosion rate than the external wall. In the two media, impedance diagrams are described by a high-frequency loop characteristic of a protective corrosion layer and a diffusion impedance loop at low frequencies associated with oxygen diffusion. Different models involving mass transfer through a porous layer are suggested for the inner and the external corrosion layers.     

Corrosion of nanocrystalline Ni–W alloys in alkaline and acidic 3.5 wt.% NaCl solutions

The corrosion behaviour of reverse-pulse electrodeposited nanocrystalline nickel tungsten alloys (nc Ni–W) in pH 3 and 10 3.5 wt.% NaCl solutions is investigated and analysed as a function of grain size. A potentiodynamic polarisation study reveals that the corrosion rate of nc Ni–W generally increases with the reduction of grain size in alkaline condition, but decreases with the reduction of grain size in acidic environment. Furthermore, for both environments, nc Ni–W alloys exhibit superior localised corrosion resistance than a microcrystalline Ni control specimen. Factors controlling the corrosion behaviour of these materials, including grain size, tungsten content, passivation and crystallographic texture are addressed.     

Effect of grain size and twins on corrosion behaviour of AZ31B magnesium alloy

Grain refining is a promising approach to improve mechanical properties of magnesium alloys, but how grain size and twins affect corrosion behaviour is not well understood. In this work, corrosion resistance of AZ31B alloy with different grain sizes is studied in 3.5% NaCl solution using immersion testing, evolved hydrogen gas measurement and potentiodynamic polarisation measurement. Intra-granular corrosion was predominant and the existence of twins further accelerated the corrosion. The effect of grain size was more pronounced in the corrosion of the untwinned microstructure. The corrosion rate significantly increased as the average grain size increased from 65 to 250 μm.     

Stress corrosion cracking of gas-tungsten arc welds in continuous-cast AZ31 Mg alloy sheet

AZ31 Mg alloy sheet was welded using a gas-tungsten arc (GTA) process over inserts containing 2.3–9.3 wt.% Al. The welded specimens were susceptible to SCC in distilled water, with susceptibility increasing with decreasing weld metal Al (or β particle) concentration. Primary stress corrosion cracks initiated at the weld metal–HAZ interface by stress-assisted localised dissolution and propagated through the weld and base metals by transgranular and intergranular H-assisted fracture (TG-HAF and IG-HAF) respectively. The IG fracture mode may be intrinsic to the texture imparted upon the base metal by rolling. The increase in SCC susceptibility with decreasing weld metal Al concentration is contrary to the purported roles of β particles in promoting localised corrosion and as crack nucleation sites, but corresponds with increases in weld – base metal galvanic current density and weld metal localised corrosion susceptibility.     

Erosion-corrosion of stainless steel under impingement by a fluid jet

The localised corrosion of 304L stainless steel is described by electrochemical measurements made under impact of a fluid jet of sodium chloride solution, in the absence of the entrained solid particles. Impingement by the fluid jet causes the open-circuit potential of 304L stainless steel in 0.6 M NaCl solution to increase with time until it attains the pitting potential. This is due primarily to acceleration of the cathodic reaction because the mass transfer rate of oxygen is increased under impingement by the fluid jet. Once the pitting potential has been achieved, the open-circuit potential decreases because of growth of stable pits. Fluid flow can itself thereby initiate stable pitting corrosion in open circuit.     

Corrosion monitoring of nickel-containing steels in marine atmospheric environment

Corrosion monitoring of nickel-containing steels has been performed in a natural atmospheric environment using AC impedance technique. A pair of identical comb-shape steel electrodes embedded in epoxy resin was used as a probe electrode for the corrosion monitoring. Three different probes of ordinary carbon steel, 2.5%Ni- and 5%Ni-containing steels were exposed to a natural marine atmosphere for the period of 14 months. The instantaneous corrosion rate of the steels was monitored by continuous measurements of the polarization resistance, and time of wetness of the steel surface was determined from high frequency impedance. The measurement was automatically carried out with an AC corrosion monitor placed at the exposure site, and data transmission between the exposure site and laboratory was performed through cellular phones.     

Effect of marine Pseudoalteromonas sp. on the microstructure and corrosion behaviour of 2205 duplex stainless steel

The effects of isolated marine Pseudoalteromonas sp. on the microstructure and corrosion behaviour of 2205 duplex stainless steel were investigated using electrochemical and surface analysis methods. Electrochemical studies demonstrated a negative shift in corrosion potential and an increase in corrosion current density in the presence of bacteria. EDS results showed a high concentration of chloride ions in the biofilm structure and a decrease in Cr content beneath the biofilm layer and near cracks. These results could lead to localised corrosion on metal surfaces. FESEM images illustrated the process of bacterial attachment on the metal surfaces at different exposure times.     

Bi-modal trend in the long-term corrosion of aluminium alloys

A wide range of literature data including in situ immersion, tidal, coastal atmospheric and industrial exposures is used to show that the trend for longer term corrosion of aluminium alloys is nearly always more consistent with a bi-modal model than with the classical power-law function. It is proposed the bi-modal characteristic results from the accumulation of corrosion products causing localised anoxic conditions. These permit a change from predominantly cathodic oxygen reduction to hydrogen ion reduction under anoxic autocatalytic conditions within pits. This mechanism is consistent with established theory for pitting corrosion in aluminium.     

Corrosion inhibition of Armco iron by 2-mercaptobenzimidazole in sodium chloride 3% media

The effect of 2-mercaptobenzimidazole (2MBI) on the corrosion of Armco iron in NaCl media has been investigated in relation to the concentration of the inhibitor by various corrosion monitoring techniques. Surface morphology was studied by scanning electron microscopy (SEM). Results obtained revealed that 2MBI is a good anodic inhibitor. The addition of increasing concentrations of 2MBI moves the corrosion potential towards positive values and reduces the corrosion rate. EIS results show that the changes in the impedance parameters (R_t and C_dl) with concentrations of 2MBI is indicative of the adsorption of these molecules leading to the formation of a protective layer on iron surface. The adsorption of this compound is also found to obey Langmuir’s adsorption isotherm in NaCl.     

Determination of the critical flow velocities for erosion–corrosion of passive materials under impingement by NaCl solution containing sand

Transition from passivation to erosion–corrosion at the critical flow velocity (CFV) is of interest for corrosion engineers. This transition phenomenon on 304 stainless steel and a Fe-based amorphous coating was investigated under impingement by sand-containing NaCl solution. Mass loss measurement, open-circuit potential monitoring and potentiostatic test were employed to determine the CFV. The CFV values defined via the three approaches are consistent, about 10 m/s for 304 stainless steel and 15 m/s for the coating. The CFV decreases with increasing the sand concentration. It may serve as an effective tool for evaluating the erosion–corrosion resistance of passive materials.