Investigating accuracy of the Tafel extrapolation method in HCl solutions

This paper discusses the validity and accuracy of the Tafel extrapolation method for determining corrosion rates of carbon steel in 1, 2 and 3 M HCl solutions open to air. Corrosion rates obtained from polarization experiments were compared with that of weight loss method. For analysing data obtained from polarization experiments electrochemical impedance spectroscopy (EIS) measurements were performed before polarization experiments. The results showed that formation of a corrosion product film and increasing the polarization resistance (R_p) of this film with time cause the corrosion rates obtained from Tafel extrapolation tend to be higher than corrosion rates obtained from weight loss test.     

Validation of corrosion rates measured by the Tafel extrapolation method

This paper discusses the validity and limitations of the Tafel extrapolation method for the determination of corrosion rates for activation-controlled corrosion processes. Experimental corrosion rates determined by the Tafel method are compared with corrosion rates obtained by an independent chemical (i.e., non-electrochemical) method for iron in hydrochloric acid, iron in 3.5% NaCl, and for titanium in boiling 1 M sulfuric acid. The methods of confirmation of the corrosion rates involved colorimetric analysis of the dissolved cation or the use of an ion-implanted inert Xe marker. Additional examples taken from the literature involving other metals and other methods of validation are also discussed.     

The in vivo and in vitro corrosion of high-purity magnesium and magnesium alloys WZ21 and AZ91

Corrosion was studied in vitro in Nor’s solution (CO₂ – bicarbonate buffered Hank’s solution) at 37 °C, and in vivo implanted in the lower back muscle of rats. Nor’s solution is a good model for HP Mg and WZ21, because (i) the pH is maintained by the same buffer as in blood and (ii) concentrations of corrosive chloride ions, and other inorganic constituents, are similar to those in blood. The higher in vitro corrosion rate of AZ91 was caused by micro-galvanic from second phases. The lower in vivo corrosion rate of AZ91 was tentatively attributed to suppression of micro-galvanic corrosion by tissue encapsulation.     

Atmospheric corrosion of magnesium alloys AZ31 and AZ61 under continuous condensation conditions

This paper studies the corrosion rate of magnesium alloys AZ31 and AZ61 exposed in humid air under continuous condensation conditions. The shape of the gravimetric curves for corrosion progress suggests that the process is controlled by factors related with the corrosion product layer growing on the metallic surface according to gravimetric results there is an initial period in which only a small part of the corroded metal is incorporated in the corrosion product layer, but after longer testing times the proportion of metal that comes to form part of this layer tends to increase very significantly.     

Influence of chloride ion concentration and temperature on the corrosion of Mg-Al alloys in salt fog

The influence of temperature and chloride concentration on the corrosion behaviour of Mg-Al alloys exposed to salt fog was evaluated. Corrosion attack increased with decreasing aluminium content in the alloy and increasing Cl⁻ concentration and temperature. The effect of Al-Mn inclusions, which revealed several stoichiometries and were up to 300 mV more noble than the magnesium matrix, was only noticeable in the early stages of corrosion of the AZ31 alloy. Aluminium segregation and β-phase distribution were the main controlling factors for the AZ80 and AZ91D alloys, the latter being more susceptible to variations in the saline concentration.     

Some particularities of the corrosion behaviour of Mg-Zn-Mn-Si-Ca alloys in alkaline chloride solutions

The influence of silicon and calcium on corrosion behaviour of Mg-6Zn-Mn alloys was investigated in alkaline solutions. The corrosion behaviour of the alloys with varied silicon and calcium content mainly depends on volume fractions and morphology of the intermetallics. In Cl-ion-free solutions silicon increased the corrosion resistance of alloys causing the growth of the surface passive film mostly composed of hydroxide products. Calcium had caused trouble to the formation of the stable protective film through the formation of Ca-Mg-Si intermetallics. In Cl-ion containing solutions intermetallic Ca-Mg-Si phase was responsible for extreme initial corrosion of Mg-6Zn-Mn-Si-Ca alloys.     

Atmospheric corrosion of field-exposed AZ91D Mg alloys in a polluted environment

The atmospheric corrosion behaviour of cast and high pressure die-cast AZ91D alloy in a polluted environment were investigated by SEM, XRD and FTIR. The atmospheric corrosion rate of the ingot was higher than that of the die-cast specimen. SO₂ gas played an important role in atmospheric corrosion. The effects of dew condensation on the ingot were greater than that on the die-cast specimen. The corrosion was initiated in the less noble α phase of the samples. The β phase, on the other hand, remained and acted as a barrier to corrosion.     

The influence of zirconium additions on the corrosion of magnesium

Sixteen custom binary Mg–Zr alloys and four commercial Zr-containing Mg-alloys were used to investigate the role of Zr on the corrosion of Mg. Mg–Zr alloys were manufactured with a range of different Zr concentrations. It was observed that the Mg–Zr alloys with a smaller mean Zr particle size had more Zr dissolved in solid solution. Both the Zr in solid solution and in metallic particle form were observed to have a deleterious effect on the corrosion rate of Mg. However, this deleterious effect is less pronounced to effect in alloys with multiple alloying additions.     

Comparison of the corrosion behaviour of bulk and thin film magnesium alloys

The corrosion behaviour of bulk and thin film specimens consisting of commercial magnesium alloys AZ31 and AZ80 was investigated by electrochemical tests in 3.5 wt.% NaCl solution and in phosphate buffered saline solution (PBS) at room temperature. Corrosion tests indicated that the corrosion rates are nearly equal, but localized attack was more pronounced for the bulk specimens than for the thin film samples. XRD analyses showed that this behaviour can be attributed to the fact that the as-deposited thin film specimens do not contain any precipitates, which are present in the bulk samples and are considered to enhance localized corrosion.     

About some corrosion mechanisms of AZ91D magnesium alloy

The present work is dedicated to a study of the corrosion resistance of AZ91D (91% Mg) alloy in wet environments. Three industrial alloys obtained by die-casting or sand casting were subjected to salt spray corrosion tests (ASTM-B117 standard) and immersion tests. Weight loss kinetic curves were measured. Surface analysis was performed by X-ray photoelectron diffraction (XPS). After corrosion the sand cast alloy presents a surface mainly enriched in hydroxides and carbonates while the die-cast alloy presents a surface enriched also in mixed Mg-Al oxides. The quantitative analysis of the rate Mg/Al shows an enrichment in aluminium for the die-cast alloys in comparison to the sand cast alloy.     

Magnesium secondary alloys: Alloy design for magnesium alloys with improved tolerance limits against impurities

The development of secondary magnesium alloys requires a completely different concept compared with standard alloys which obtain their corrosion resistance by reducing the levels of impurities below certain alloy and process depending limits. The present approach suitable for Mg-Al based cast and wrought alloys uses a new concept replacing the β-phase by τ-phase, which is able to incorporate more impurities while being electro-chemically less detrimental to the matrix. The overall experimental effort correlating composition, microstructure and corrosion resistance was reduced by using thermodynamic calculations to optimise the alloy composition. The outcome is a new, more impurity tolerant alloy class with a composition between the standard AZ and ZC systems having sufficient ductility and corrosion properties comparable to the high purity standard alloys.     

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.     

Effects of organic acid pickling on the corrosion resistance of magnesium alloy AZ31 sheet

Organic acids were used to clean AZ31 magnesium alloy sheet and the effect of the cleaning processes on the surface condition and corrosion performance of the alloy was investigated. Organic acid cleanings reduced the surface impurities and enhanced the corrosion resistance. Removal of at least 4 μm of the contaminated surface was required to reach corrosion rates less than 1 mm/year in salt spray condition. Among the three organic acids examined, acetic acid is the best choice. Oxalic acid can be an alternative while citric acid is not suitable for cleaning AZ31 sheet, because of insufficient removal of iron impurities.     

Influence of inorganic acid pickling on the corrosion resistance of magnesium alloy AZ31 sheet

Surface contaminants as a result of thermo-mechanical processing of magnesium alloys, e.g. sheet rolling, can have a negative effect on the corrosion resistance of magnesium alloys. Especially contaminants such as Fe, Ni and Cu, left on the surface of magnesium alloys result in the formation of micro-galvanic couples and can therefore increase corrosion attack on these alloys. Due to this influence they should be removed to obtain good corrosion resistance.In this study, the effect of inorganic acid pickling on the corrosion behaviour of a commercial AZ31 magnesium alloy sheet was investigated. Sulphuric, nitric and phosphoric acids of different concentrations were used to clean the alloy for various pickling times. The surface morphology, composition and phases were elucidated using scanning electron microscopy, X-ray fluorescence analysis, spark discharge-optical emission spectroscopy, energy dispersive X-ray spectroscopy and infrared spectroscopy. The effect of surface cleaning on the corrosion properties was studied using salt spray test and electrochemical impedance spectroscopy. The experimental results show that acid pickling reduces the surface impurities and therefore enhances the corrosion resistance of the alloy. The cleaning efficiency of the three acids used and the corrosion protection mechanisms were found to be remarkably different. Best corrosion results were obtained with nitric acid, followed closely by phosphoric acid. Only the sulphuric acid failed more or less when cleaning the AZ31 sheet. However, to obtain reasonable corrosion resistance at least 5 μm of the surface of AZ31 magnesium alloy sheet have to be removed.     

Effect of antimony, bismuth and calcium addition on corrosion and electrochemical behaviour of AZ91 magnesium alloy

This study investigated the effect of antimony, bismuth and calcium addition on the corrosion and electrochemical behaviour of AZ91 magnesium alloy in 3.5% NaCl solution. Techniques including constant immersion, electrochemical potentiodynamic polarisation, scanning electron microscopy (SEM), energy dispersed spectroscopy (EDS) and X-ray diffraction (XRD) were used to characterise electrochemical and corrosion properties and surface topography. It was found that corrosion attack occurred preferentially on Mg₃Bi₂ and Mg₃Sb₂ particles while Mg₁₇Al₈Ca_0.5 and Mg₂Ca phases showed no detrimental effect on corrosion. Combined addition of small amounts of bismuth and antimony to the AZ91 alloy resulted in significant increase in corrosion rate.     

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.     

An exploratory study of the corrosion of Mg alloys during interrupted salt spray testing

A first systematic investigation was carried out to understand the corrosion of common Mg alloys (Pure Mg, AZ31, AZ91, AM30, AM60, ZE41) exposed to interrupted salt spray. The corrosion rates were also evaluated for these alloys immersed in 3 wt.% NaCl by measuring hydrogen evolution and an attempt was made to estimate the corrosion rate using Tafel extrapolation of the cathodic branch of the polarisation curve. The corrosion of these alloys immersed in the 3 wt.% NaCl solution was controlled by the following factors: (i) the composition of the alpha-Mg matrix, (ii) the volume fraction of second phase and (iii) the electrochemical properties of the second phase. The Mg(OH)₂ surface film on Mg alloys is probably formed by a precipitation reaction when the Mg²⁺ ion concentration at the corroding surface exceeds the solubility limit. Improvements are suggested to the interrupted salt spray testing; the ideal test cycle would be a salt spray of duration X min followed by a drying period of (120-X) min. Appropriate apparatus changes are suggested to achieve 20% RH rapidly within several minutes after the end of the salt spray and to maintain the RH at this level during the non-spray part of the cycle. The electrochemical measurements of the corrosion rate, based on the “corrosion current” at the free corrosion potential, did not agree with direct measurements evaluated from the evolved hydrogen, in agreement with other observations for Mg.     

Testing validity of the Tafel extrapolation method for monitoring corrosion of cold rolled steel in HCl solutions - Experimental and theoretical studies

The protection influence of glycine (Gly) and a one of its derivatives, namely 2-(bis(2-aminoethyl)amino) acetic acid, designated here as GlyD; where GlyD stands for “glycine derivative”, against cold rolled steel (CRS) corrosion was studied in aerated stagnant 1.0 M HCl solutions at 25 °C. Measurements were conducted under various experimental conditions using Tafel polarization, linear polarization and impedance techniques. These studies have shown that Gly and GlyD are very good “green”, mixed-type inhibitors. GlyD is more effective than Gly itself in inhibiting the acid corrosion of CRS. Electrochemical frequency modulation (EFM) and inductively coupled plasma-atomic emission spectroscopy (ICP-AES) method of analysis are also presented here for monitoring corrosion. Corrosion rates obtained from both EFM and ICP-AES methods are comparable with those recorded using Tafel extrapolation method, confirming validation of corrosion rates measured by the latter. Adsorption via H-bond is discussed here, based on the presence of oxide film on the electrode surface as well as the number of NH linkages in the inhibitor molecule. Quantum chemical method was also employed to explore the relationship between the inhibitor molecular properties and its protection efficiency. The density function theory (DFT) is used to study the structural properties of Gly and GlyD in aqueous phase in an attempt to understand their inhibition mechanism. The protection efficiencies of these compounds showed a certain relationship to highest occupied molecular orbital (HOMO) energy, Mulliken atomic charges and Fukui indices.     

The influence of the microstructure on the atmospheric corrosion behaviour of magnesium alloys AZ91D and AM50

Even though magnesium, as a structure metal, is most commonly used in an atmospheric environment, most investigations of magnesium are performed in solution. In the present work the atmospheric corrosion of two commonly used magnesium alloys, AZ91D and AM50, has been investigated from the initial stages up to the most severe forms of corrosion. A detailed investigation of the morphology of a corrosion attack and its development over time shows that the atmospheric corrosion mechanism is similar for the two alloys. Based on these findings a schematic model of the initial atmospheric corrosion attack on AZ91D is presented and discussed.