Corrosion Patterns Research Articles

Propagation of corrosion and corrosion patterns of bars embedded in RC beams stored in chloride environment for various periods

In this paper, the propagation of chloride-induced corrosion of bars embedded in reinforced concrete (RC) beams exposed to a chloride environment was examined. These bars were exposed to chloride environment for various periods. Then, the cracking of the RC beams was observed and the corroded reinforcements were extracted from the beams in order to investigate the corrosion distribution and propagation. The cross-sectional loss and spatial distribution in the longitudinal direction were studied by cutting the residual reinforcement into several small specimens. By analysing the cross-sectional loss results, it was found that the corrosion developed in a stochastic way. However, the corrosion zone developed gradually from the mid-span of the beam to the two ends, and there was a linear relationship between the extent of the corrosion zone and the corrosion period. Similarly, the degree of pitting corrosion and general corrosion also increased. The relationship between pitting corrosion and general corrosion developed along an improved exponential curve, while the pitting factor and the general corrosion showed a logarithmic relationship. The results of this investigation could be treated as an envelope curve, which can be helpful to assess the corrosion state of reinforcements situated in aggressive environment and predict the residual life-cycle of corroded constructions.

The role of rope research in a safe process operation of cableways

The article presents the problems connected with the dynamic development of the cableways in Poland in recent years. The authors characterize ropeways which most often are run in Poland. Authors focus on the role of obligatory testing in the operation of ropeways. They pay most attention to steel wire ropes and some elements cooperating with them. The article discusses the main causes of wear of ropes and mechanisms of damage such as fatigue, wear and different corrosion patterns. The examples of such damage and the place in which they can occur are presented. Main attention was placed on visual assessment of damage caused by corrosive wear.

Corrosion behavior of TiC–SiC composite ceramics in molten FLiNaK salt

Immersion corrosion tests of TiC0.8, TiC, TiC–20vol% SiC, TiC–40vol% SiC and SiC have been performed in molten FLiNaK salt at 800°C for 25–200h under argon cover gas. All of these five samples showed small mass loss and relatively good corrosion resistance in molten FLiNaK salt. The corrosion patterns of TiC0.8, TiC, TiC–20vol% SiC and TiC–40 vol% SiC were inter-granular corrosion, which were attributed to the depletion of Ti along the grain boundaries. SiC exhibited a general corrosion process in which a carbon-rich layer formed on the surface, resulting from the depletion of Si. The carbon-rich layer protected SiC against further corrosion, hence lowering the corrosion rate. The corrosion results of TiC–20% SiC and TiC–40% SiC revealed the corrosion resistance of TiC could be improved by adding SiC. And the contribution of SiC to better corrosion resistance has been elucidated.

Investigation of corrosion pattern of AISI 316 and AISI 304 stainless steels in Na2B4O7·5H2O solution

Investigation of corrosion pattern of AISI 316 and AISI 304 stainless steels in Na2B4O7·5H2O solution

Development and Assessment of a New Flow-Through Test Instrument to Study Wear and Erosion Effects of Nanofluids

Nanofluids, the suspensions of nano-size powders in ordinary fluids, are of technical interest for their enhanced cooling properties, but their possible erosion-corrosion effects on cooling-system materials are mostly unknown. This paper discusses the rationale for designing and developing a new test-rig with flow-through parallel to the tested surfaces. The instrument conduct-chamber accommodates multiple specimens for simultaneous testing, and controlled fluid speed and temperature. This study shows that the new rig yields measurable surface-modifications from nanofluid action in reasonable test-times. Results are presented for a nanofluid (of 2%-alumina-nanopowder in water) that is recirculated in parallel-flow contact with polished aluminum and copper. Surface modifications are assessed by roughness, weighing of removed-material, and optical-microscopy, and results indicate that nanopowders can lead to patterns of wear, erosion and corrosion that are substantially different than those typically obtained from the base-fluids.

Fretting corrosion behavior of nitinol spinal rods in conjunction with titanium pedicle screws

Untypical corrosion damage including erosions combined with the build-up of titanium oxide as a corrosion product on the surface of explanted Nitinol spinal rods in the areas where it was in contact with titanium pedicle screw head is reported. It was suggested that Nitinol rods might have inferior fretting corrosion resistance compared with that made of titanium or CoCr.Fretting corrosion of Nitinol spinal rods with titanium (Ti6Al4V) pedicle screws were tested in-vitro by conducting a series of potentiostatic measurements of the peak-to-peak values of fretting corrosion current under bending in a 10% solution of calf serum in PBS. The test included Nitinol rods locked in titanium pedicle screws of different designs. Performance of commercially available titanium (Ti6Al4V) and CoCr spinal rods was also investigated for a comparison. Corrosion damage observed after the in-vitro tests was studied using SEM and EDAX analysis and was compared with patterns on Nitinol rods retrieved 12months after initial surgery. Metal ions level was measured in the test media after in-vitro experiments and in the blood and tissues of the patients who had the rods explanted.The results of this study revealed that Nitinol spinal rods locked in Ti pedicle screws are susceptible to fretting corrosion demonstrating higher fretting corrosion current compared with commercially used Ti6Al4V and CoCr rods. On the surface of Nitinol rods after in-vitro tests and on those retrieved from the patients similar corrosion patterns were observed. Improved resistance to fretting corrosion was observed with Nitinol rods in the in-vitro tests where pedicle screws were used with a stiffer locking mechanism. Since the development of the localized corrosion damage might increase the risk of premature fatigue failure of the rods and result in leaching of Ni ions, it is concluded that Nitinol rods should not be used in conjunction with Ti pedicle screws without special protection especially where the design provides a high degree of mobility to the rods.

Inflammatory cell-induced corrosion in total knee arthroplasty: A retrieval study.

Metal release in patients with joint replacements is associated with local tissue reactions, pain, and ultimately revision of implants. One of the causes of this metal loss is speculated to be due to a mechanism of inflammatory cell-induced corrosion (ICIC). In this knee retrieval study, we aimed to: (1) identify the extent and location of ICI corrosion patterns on our femoral and tibial components and (2) correlate our findings with implant and clinical information. We investigated 28 femoral and 9 tibial components made of polished CoCr for presence of ICIC, using macroscopic and microscopic screening and statistical analyses to identify any significant correlations between our results and clinical information. We found that 71% of femoral and 100% of tibial components showed evidence of ICIC and significantly more was present on non-contacting regions (p < 0.0001). We found a significant correlation between the presence of ICIC and instability (p = 0.0113) and a significant difference between poster stabilized and cruciate retaining designs in the amount of ICIC on internal edges (p = 0.0375). This corrosion pattern was prevalent in our series of knee retrievals and may help explain some of the mechanisms of material loss that may occur in vivo. © 2016 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 106B: 460-467, 2018.

Probabilistic physical modelling of corroded cast iron pipes for lifetime prediction

Cast iron was the dominant material for buried pipes for water networks prior to the 1970s in Australia and overseas. At present, many water utilities still have a significant amount of ageing cast iron pipes. Cast iron is a brittle material and when large diameter cast iron pipes (diameters above 300mm) further deteriorate, the consequences of failure can be substantial. Focusing on the likelihood of failure to assist risk assessment, this paper examines the performance of large-diameter cast iron pipes using probabilistic analysis, incorporating uncertainties of governing variables. Finite element analysis is first conducted to study the physical mechanism of buried pipes subjected to complex environmental conditions. The deterioration of cast iron pipes due to corrosion is considered on the basis of recent research. The uncertainties of governing variables, such as the physical properties of soil, cast iron, water pressure and corrosion patterns, in pipe failure risk assessment are considered. Using probabilistic physical modelling, the lifetime probability of failure is derived and a time-dependent sensitivity analysis is presented. The results of this probabilistic physical modelling are compared with cohorts of failure data from two Australian water utilities to examine the underlying trends from both physical modelling and statistical analysis.

Buckling Analysis of Corroded Angle Beams with Irregular Random Surfaces

Yielding, excessive deformation, brittle fracture, fatigue, and elastic buckling are different modes of failure of structures. Buckling or instability occurs in structures under compressive stresses. Corrosion is a time-dependent degradation which reduces load-bearing capacity. This article investigates the detrimental effect of corrosion on reduction of elastic buckling strength of angle beams with irregular random surfaces. Different corrosion patterns and corrosion parameters are considered. Comparing the results with those available in the literature, it indicates that simulation technique used in this study has good accuracy and applicability for simulation of irregular random surfaces of corroded structures. It also shows that neglecting irregular random surface of corroded angle beams could leads up to 6% overestimation of buckling load. It is found that irregularity of surface has little influence on reduction of buckling load when roughness is low, however, for high values of roughness, the more roughness of surface the lower buckling strength of angle beams.

The monitoring of galvanic corrosion behaviour caused by mineral deposit in pipeline working conditions using ring form electronic resistance sensor system

In this paper, a ring form electronic resistance (ER) sensor system was developed for the study of under-deposit corrosion (UDC) caused by mineral deposit in pipeline conditions. The corrosion status of different locations along the pipeline circle direction and the galvanic current between the deposit-covered area and the bare steel area were monitored by the conjunction of the ER method and electrochemical noise technique. The performance of inhibitor ethylene diamine tetra methylene phosphonic acid sodium (EDTMPS) on UDC was studied by both the electrochemical method and the ring form ER sensor system. The measurement results demonstrated that the inhomogeneous distribution of the rust layer will cause heterogeneous corrosion patterns in the pipeline. Though EDTMPS showed an excellent inhibition effect for the corrosion of both deposit-covered steel and bare steel in electrochemical tests, it failed for the inhibition of UDC in the pipeline working conditions and serious localised corrosion beneath the sand particle was monitored by the sensor system.

Investigating effects of potential excursions and pH variations on cathodic protection using new electrochemical testing cells

Excursion from safe cathodic protection (CP) potentials occurs on buried steel pipelines due to various forms of electrical interferences such as stray currents. Variations in pH can also occur over some pipeline sections such as seashore and river crossing pipes. Currently, the exact effects of potential excursion and the pH on CP efficiency have not been sufficiently quantified preliminary due to difficulties in measuring these effects. In this work, these effects have been investigated using electrochemical cells designed to mimic the high resistivity and pH conditions observable over underground steel pipes, including a new electrochemical cell that has been designed to facilitate the effective simulation and control of pH, potential excursion and other CP testing parameters. The pH has been shown to be a key factor affecting the patterns of corrosion and CP efficiency. Localised corrosion has been found to be the dominating form of corrosion under potential excursions conditions.

Clinical relevance of corrosion patterns attributed to inflammatory cell induced corrosion: A retrieval study

Event Abstract Back to Event Clinical relevance of corrosion patterns attributed to inflammatory cell induced corrosion: A retrieval study Anna Di Laura1*, Harry Hothi1*, Jay Meswania1*, Robert Whittaker1*, Danielle De Villiers1*, Gordon Blunn1, John Skinner1* and Alister Hart1* 1 University College London, Institute of Orthopaedics and Musculoskeletal Science (IOMS), United Kingdom Introduction: Cobalt Chromium (CoCr) alloys are the most commonly used materials in orthopaedics. In vitro studies have shown that human osteoclasts can corrode Stainless Steel and Titanium forming resorption pits that lead to the production of metal ions responsible for inflammatory reactions [1],[2]. Moreover, traces of cellular activities on metal orthopaedic explants have recently been reported as Inflammatory Cell-Induced (ICI) corrosion being the result of the cells sealing on the metal surfaces and releasing Reactive Oxygen Species through Fenton-like reactions [3]. We aimed to better understand the clinical relevance of this phenomenon. Our objectives were 1) to determine the extent of ICI corrosion patterns observed in a large cohort of retrieved hip implants,2) to give a 3Dimensional characterisation to the corroded regions, 3) to correlate our findings with clinical data. Materials: This was a retrieval study involving 100 failed hip implants received at our centre with metal-on-metal (MOM) bearings. We selected Durom modular heads [n=25], Durom resurfacing [n=25] as well as ASR modular heads [n=25] and ASR resurfacing [n=25] for inspection, so that we included both metallurgical features for Co-Cr alloys: wrought and cast. Methods: Initial screening included macroscopic and microscopic analysis of both cups and heads with the aid of a stereomicroscope (Leica) and a digital microscope (Keyence). A scanning electron microscope (Jeol) was used to perform detailed analysis of specific areas of interest highlighted from the initial visual screening. Second and Backscattered Electron Imaging ranging from low (4-5) to high (10-15) kV were used. Energy dispersive X-ray spectrometry (EDS) assessed the elemental composition and organic biocorrosion products on the implant surface. A 3D optical microscope (Bruker) showed both depth of pits and deposition of material in µm.The significance of any univariable associations between the presence of ICI corrosion and key implant and clinical variables were assessed using simple linear regression models. Results: 59 % of implants had evidence of corrosion patterns as described by Gilbert et al. Haziness, pits and streaks were visible on the: polished regions of the heads as well as on internal edges of cups; and machining marks of acetabular rims and head rims (Figure1). Higher magnification images showed crater-like spots surrounded by a frosted pattern of a ruffled topography. SEM images confirmed the presence of the features probably derived from different cellular mechanisms . Further confirmation of this was achieved by the optical profilometer. Scaled maps gave information about depth of pits that ranged from 0.5 to 3µm. There was a significant association between ICI corrosion and aseptic loosening for the ASR modular (r=-0.488, p=0.016) and the Durom modular (r=0.454, p=0.026) explants. EDS analysis revealed the presence of iron in correspondence of corroded regions, the element was only detectable in proximity of the corrosion patterns. Discussion: Corrosion patterns potentially attributable to cell induced corrosion were found on 59% of retrieved hips. There was no difference between cast and wrought microstructures but there was a correlation between implants revised due to aseptic loosening and presence of cellular corrosion. Iron is also in accordance with the presence of phagocytic cells on the metal surfaces due to Fenton-like reaction taking place. We recommend further work to determine the role and type of cells in the corrosion damage patterns observed. This may help understand which patient factors are involved in hip implant failure: and whether some patients more prone to failure of CoCr implants?

Mathematical Model of Homogeneous Corrosion of Steel Pipe Pile Foundation for Offshore Wind Turbines and Corrosive Action

In this paper, the nonlinear corrosion model under the combined action of the anticorrosion system and corrosive environment is chosen as the mathematical model of homogeneous corrosion of steel pipe pile foundation for the offshore wind turbine. Based on the mathematical model, a three-dimensional finite element model was established for the steel pipe pile foundation of the offshore wind turbine. And the homogeneous corrosion action of the steel pipe piles was calculated, and the reduction rules of the strength and stability of the steel pipe piles for wind turbines under different corrosion patterns are analyzed. According to the calculation results, the mathematical model can be used in the analysis of corrosion for steel pipe pile in the wind turbine. Under the normal operation conditions, the reduction rules of the strength and stability of the steel pipe piles contain three stages: no influence stage, negative exponential decrease stage, and stable stage. But under the extreme load conditions, the effect of corrosion is enormous for the strength and stability of the steel pipe pile.

The use of multiple pseudo-physiological solutions to simulate the degradation behavior of pure iron as a metallic resorbable implant: a surface-characterization study.

Understanding the interactions of a pure iron surface with biological elements, such as ions and proteins in an aqueous medium, is essential for an accurate in vitro assessment of corrosion patterns. In fact, the synergy of chlorides, carbonates, phosphates and complex organic molecules present in the body environment is a key factor affecting both in vivo and in vitro degradation of materials, especially iron and its alloys. The aim of this work was the assessment of degradation patterns of pure iron in 5 commercial pseudo-physiological solutions by a thorough study of degraded surface chemistry and morphology. It also provides a methodological basis to understand the short-term degradation mechanism of degradable iron depending on the surrounding physiological media. The standard static immersion corrosion test was modified to adapt the procedure to pseudo-physiological solutions. After a 14-day static immersion test, the surfaces of samples were investigated by scanning electron microscopy, stylus profilometry and atomic force microscopy techniques. The chemistry and phase composition of the degraded layers were evaluated, respectively, by X-ray photoelectron spectrometry and X-ray diffractometry. The morphology and composition of the degradation layers were found to be different for the test-solutions: for phosphate-rich solutions, the formation of an adherent passive layer was found; degradation mechanisms related to general corrosion were predominant for all the other solutions. In conclusion, the chemical composition of the used medium plays a fundamental role in the degradation pattern of pure iron, so that direct comparisons of solutions with different ion concentrations, as reported in the literature, need to be carefully assessed.

Effect of Annealing Heat Treatment to Characteristics of AlDC8 (Al-Si-Cu) Alloy

(Received January 20, 2015; Revised November 09, 2015; Accepted November 10, 2015)ALDC8 (Al-Si-Cu) alloy has been often corroded with pattern of intergranular corrosion in corrosive environments. Thus, in order to improve its corrosion resistance, the effect of annealing heat treatment to corrosion resistance and hardness was investigated with parameters of heating temperatures such as 100 ℃, 200 ℃, 300 ℃, 400 ℃ and 500 ℃ for 1hr. The hardness was varied with annealing temperature and slightly decreased with annealing heat treatment. However, the relation between annealing temperature and hardness agreed not well each other. Corrosion potential was shifted to noble direction and corrosion current density was also decreased with increasing annealing temperature. Moreover, both AC impedance at 10 mHz and polarization resistance on the cyclic voltammogram curve were also increased with increasing annealing temperature. Furthermore, intergranular corrosion was somewhat observed in non heat treatment as well as annealing temperatures at 100 ℃, 200 ℃ and 300 ℃, while, intergranular corrosion was not nearly observed at annealing temperature of 400 ℃, 500 ℃. Consequently, it is considered that the annealing heat treatment of ALDC8 alloy may be an available method not only to inhibit its intergranular corrosion but also to improve its corrosion resistance.Keywords: intergranular corrosion, annealing heat treatment, cyclic voltammogram, AC impedance, corrosion current density

셀 몰드 주조한 SSC13 엘보우 피팅 주강의 고용화율에 따른 델타 페라이트 분율 변화와 내부식특성

In this study, the measurement of FN (ferrite volume fraction) and the solution annealing ratio at a temperature of were determined with 15A elbow fittings of shell cast SSC13, and the corrosion resistance with and without austenitizing solution annealing were investigated in comparison with AISI304. The delta ferrite phase was observed in the material due to the slow cooling effect of the shell mold casting. However, the delta ferrite phase decreased gradually with the solution annealing at a temperature of . The hardness generally decreased with a heat treatment; however, its corrosion resistance was improved with the heat treatment. In addition, when a passivation treatment was applied, its corrosion ratio showed the lowest value. The pattern of general corrosion decreased due to the decrease in the delta ferrite phase with the solution annealing treatment. Consequently, it is suggested that the corrosion resistance of SSC13 elbow fittings can be improved by increasing the ratio of any solution annealing treatment used and by decreasing the ferrite phase. The relationship between the ratio of solution annealing and delta ferrite is expressed as follows: SA (solution annealing ratio,%) = 98 - FN (ferrite volume fraction, %).

Experimental study on stray current corrosion of coated pipeline steel

The experimental investigation on the coated pipeline steel was performed with respect to induced stray current including the alternating current (AC), direct current (DC), and the superimposed AC + DC sources in the soil-simulating conditions. The results showed that the corrosion potential and rate increased with the increases of stray current density. The extent of corrosion was found to be less in AC when compared to the DC sources. When an alternating current was superimposed on a direct current, the corrosion rate was larger than their respective corrosion. AC corrosion on the X70 pipeline steel represented three classic patterns, including the uniform corrosion, ring-like corrosion and pitting corrosion according to the geometrical shape of the corrosion images. The corrosion pattern would transfer to the local corrosion from the uniform corrosion, when the AC density increased. The quantity, area and depth of the corrosion pits arose with the increase of the AC current density. The relationship between the pit number or pit area and AC current density followed the power function, y = aib.

Clinical relevance of corrosion patterns attributed to inflammatory cell-induced corrosion: A retrieval study.

In vitro studies have shown that human osteoclasts can corrode stainless steel and titanium leading to the production of metal ions responsible for inflammatory reactions. Moreover, traces of cellular activities on metal orthopaedic explants have recently been reported as inflammatory cell-induced (ICI) corrosion being the result of the cells sealing on the metal surfaces and releasing reactive oxygen species (ROS) through Fenton-like reactions. The extent and clinical relevance of this phenomenon has yet to be understood. We analysed a cohort of 100 CoCr alloy hips collected at our retrieval centre; we performed macroscopic and microscopic screening and used statistical analysis to correlate our findings with implant and clinical variables. We found that 59% of our implants had evidence of surface damage consistent with what has previously been described as cell-induced corrosion. There was a significant association between the patterns and aseptic loosening for the ASR modular (r = -0.488, p = 0.016) and the Durom modular (r = 0.454, p = 0.026). This is the largest implant retrieval study to examine the phenomena of so-called ICI corrosion and is the first to investigate its clinical relevance. We recommend further work to determine the role of cells in the damage patterns observed. © 2015 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 105B: 155-164, 2017.

Monitoring corrosion of reinforced concrete beams in a chloride containing environment under different loading levels

Corrosion has significant adverse effects on the durability of reinforced concrete (RC) structures, especially those exposed to a marine environment and subjected to mechanical stress, such as bridges, jetties, piers and wharfs. Previous studies have been carried out to investigate the corrosion behaviour of steel rebar in various concrete structures, however, few studies have focused on the corrosion monitoring of RC structures that are subjected to both mechanical stress and environmental effects. This paper presents an exploratory study on the development of corrosion monitoring and detection techniques for RC structures under the combined effects of external loadings and corrosive media. Four RC beams were tested in 3% NaCl solutions under different levels of point loads. Corrosion processes occurring on steel bars under different loads and under alternative wetting - drying cycle conditions were monitored. Electrochemical and microscopic methods were utilised to measure corrosion potentials of steel bars; to monitor galvanic currents flowing between different steel bars in each beam; and to observe corrosion patterns, respectively. The results indicated that steel corrosion in RC beams was affected by local stress. The point load caused the increase of galvanic currents, corrosion rates and corrosion areas. Pitting corrosion was found to be the main form of corrosion on the surface of the steel bars for most of the beams, probably due to the local concentration of chloride ions. In addition, visual observation of the samples confirmed that the localities of corrosion were related to the locations of steel bars in beams. It was also demonstrated that electrochemical devices are useful for the detection of RC beam corrosion.

The Use of EDTA As a Mechanistic Probe of Magnesium Corrosion in Chloride Containing Electrolyte

Despite recent developments which have led to a better understanding of the localized corrosion behavior of magnesium (Mg) in chloride electrolyte, there are certain characteristics which remain unclear. One such aspect is the enhanced electro-catalytic activity of the dark corroded Mg surface to cathodic hydrogen evolution, previously demonstrated in several recent publications. It has also been shown that in many instances, the time-dependent Mg corrosion rate increases in proportion to the area of cathodically- activated dark surface produced in the early stages of localized corrosion propagation. It has been proposed that the effect is due to enrichment in transition metal impurity phases on the anodically attacked Mg surface. However, direct evidence of such enrichment has proved difficult, although recent reports have located iron-rich particles within the corrosion product layer (1) and interfacial Fe enrichment on a corroded Mg by Rutherford backscattering spectroscopy (2). In this work the localized corrosion of commercial purity Mg, comprising a 280 ppm Fe impurity level, is investigated in the presence and absence of ethylene diamine tetra-acetic acid (EDTA), at different concentrations and pH values, when immersed in aqueous sodium chloride electrolyte. The aim of the investigation is to exploit the ability of EDTA to act as a chelating agent for Mg2+ and transition metal ions such as Fe2+, so that the principal influencing factors causing the propagation of cathodically activated dark regions on corroding Mg may be identified. The presence of EDTA in 1 mol dm-3 NaCl (aq) at pH 6 is shown to markedly decrease the rate of Mg corrosion in freely corroding conditions, as shown by the plots of time dependent hydrogen evolution (HE) given in Figure 1. Increasing the concentration of EDTA progressively reduces the rate of HE and produces a change in kinetics, where a monotonic increase in the quantity of hydrogen evolved with time is observed. This is in contrast to the exponential relationship seen over the first 60 min immersion for the control experiment and at a lower EDTA concentration. The decrease in corrosion rate is accompanied by a significant change in the visual appearance of the corroding surface. Although dark, expanding circular corrosion features propagate in the presence of 10-3 mol dm-3EDTA, no such behavior is observed when the concentration is increased by an order of magnitude, and the exposed surface remains covered by a cloudy white film. In-situ SVET shows no evidence of localized corrosion patterns characteristic of the un-inhibited case even over protracted immersion times of several hours. Similar observations are observed at higher pH although the influence of EDTA on bulk rates of HE are not as marked when compared to the control case. In separate experiments, where localized corrosion is allowed to propagate before EDTA addition to the corrosive electrolyte, significant changes in current density patterns are observed at the point of addition. This is shown in Figure 2, where EDTA halts propagation and the intense local anodic activity observed at the perimeter of the corroded area becomes displaced to the remaining intact (uncorroded) surface, although at significantly lower current density values. The observations of a strongly mitigated HE rate seem to be inconsistent with a scenario where EDTA complexation of Mg2+ ions prevents deposition of surface corrosion product. A theory is proposed to explain the action of EDTA in terms of its ability to strongly chelate Fe2+, possibly present in the direct vicinity of the corroding Mg surface, in a situation where Fe-rich impurity phases become detached from the matrix and corrode independently of the matrix. The possibility of subsequent re-plating of Fe onto the corroding Mg to produce an electro-catalytically activated surface is considered. The acceleration of HE rates from corroding Mg surfaces on which iron re-plating has taken place is confirmed by experimentation.