Microstructure and grain boundary microanalysis of X70 pipeline steel

Grain boundaries (GBs), particularly ferrite: ferrite GBs, of X70 pipeline steel were characterized using analytical electron microscopy (AEM) in order to understand its intergranular stress corrosion cracking (IGSCC) mechanism(s). The microstructure consisted of ferrite (alpha), carbides at ferrite GBs, some pearlite and some small precipitates inside the ferrite grains. The precipitates containing Ti, Nb, V and N were identified as complex carbo-nitrides and designated as (Ti, Nb, WC, N). The GB carbides occurred (1) as carbides along ferrite GBs, (2) at triple points, and (3) at triple points and extending along the three ferrite GBs. The GB carbides were Mn rich, were sometimes also Si rich, contained no micro-alloying elements (Ti, Nb, V) and also contained no N. It was not possible to measure the GB carbon concentration due to surface hydrocarbon contamination despite plasma cleaning and glove bag transfer from the plasma cleaner to the electron microscope. Furthermore, there may not be enough X-ray signal from the small amount of carbon at the GBs to enable measurement using AEM. However, the microstructure does indicate that carbon does segregate to alpha : alpha GBs during microstructure development. This is particularly significant in relation to the strong evidence in the literature linking the segregation of carbon at GBs to IGSCC. It was possible to measure all other elements of interest. There was no segregation at alpha : alpha GBs, in particular no S, P and N, and also no segregation of the micro-alloying elements, Ti, Nb and V. (C) 2003 Kluwer Academic Publishers.

Understanding magnesium corrosion - A framework for improved alloy performance

The purpose of this paper is to provide a succinct but nevertheless complete mechanistic overview of the various types of magnesium corrosion. The understanding of the corrosion processes of magnesium alloys builds upon our understanding of the corrosion of pure magnesium. This provides an understanding of the types of corrosion exhibited by,magnesium alloys, and also of the environmental factors Of most importance. This deep understanding is required as a foundation if we are to produce magnesium alloys much more resistant to corrosion than the present alloys. Much has already been achieved, but there is vast scope for improvement. This present analysis can provide a foundation and a theoretical framework for further, much needed research. There is still vast scope both for better fundamental understanding of corrosion processes, engineering usage of magnesium, and also on the corrosion protection of magnesium alloys in service.

Electrochemical study of UNS S32550 super duplex stainless steel corrosion in turbulent seawater using the rotating cylinder electrode

The cathodic and anodic characteristics of freshly polished and pre-reduced UNS S32550 (ASTM A479) super duplex stainless steel in a filtered and conductivity-adjusted seawater have been investigated under controlled flow conditions. A rotating cylinder electrode was used together with both steady and non-steady-state voltammetry and a potential step current transient technique to investigate the electrode reactions in the fully characterized electrolyte. Both oxygen reduction and hydrogen evolution were highly irreversible and the material exhibited excellent passivation and repassivation kinetics. Relative corrosion rates were derived and the corrosion mechanism of the alloy was found to be completely independent of the mass-transfer effects, which can contribute to flow-induced corrosion.

Electrochemical corrosion of unalloyed copper in chloride media–a critical review

The literature dealing with the electrochemical corrosion characteristics of unalloyed copper in aqueous chloride media is examined. The enormous quantity of polarisation and mixed/corrosion potential data that has been made available in the literature over the last 50 years has been compiled and discussed in a comprehensive review. For a wide range of electrode geometries, the importance of the chloride ion and the mass transport of anodic corrosion products on the corrosion behaviour of copper are made clear for both freshly polished and 'filmed' surfaces. (C) 2003 Elsevier Ltd. All rights reserved.

Corrosion behaviour of magnesium in ethylene glycol

Corrosion of magnesium engine components by coolant is an important issue in the automotive industry where magnesium alloys may be used. It is of significance to understand the corrosion behaviour of pure magnesium in ethylene glycol solutions, as this can provide a basis for developing new coolants for magnesium alloy engine blocks. In this paper, through corrosion and electrochemical tests, it was found that the corrosion rate of magnesium decreased with increasing concentration of ethylene glycol. Individual contaminants, such as NaCl, NaHCO3, Na2SO4 and NaCl can make aqueous ethylene glycol solution more corrosive to magnesium. However, in NaCl contaminated ethylene glycol, NaHCO3 and Na2SO4 showed some inhibition effect. The solution resistivity played an important role in the corrosion of magnesium in ethylene glycol solutions, and the competitive adsorption of ethylene glycol and the contaminants on the magnesium surface was also responsible for the observed corrosion behaviours. The corrosion of magnesium in ethylene glycol can be effectively inhibited by addition of fluorides that react with magnesium and form a protective film on the surface. (C) 2003 Elsevier Ltd. All rights reserved.

Corrosion resistance of aged die cast magnesium alloy AZ91D

The corrosion behaviour of die cast magnesium alloy AZ91D aged at 160degreesC was investigated. The corrosion rate of the alloy decreases with ageing time in the initial stages and then increases again at ageing times greater than 45 h. The dependence of the corrosion rate on ageing time can be related to the changes in microstructure and local composition during ageing. Precipitation of the beta phase (Mg17Al12) occurs exclusively along the grain boundaries during ageing. The beta phase acts as a barrier, resulting in a decreasing corrosion rate in the initial stages of ageing. In the later stages, the decreasing aluminium content of alpha grains makes the alpha matrix more active, causing an increase in the corrosion rate. Electrochemical testing results also confirm the combined effects of the changes in alpha and beta phases on the corrosion resistance of the aged die cast AZ91D alloy. (C) 2003 Elsevier B.V. All rights reserved.

Analysis of service stress corrosion cracking in a natural gas transmission pipeline, active or dormant?

Stress corrosion cracks (SCC) had been found in a natural gas transmission pipeline during a dig-up and inspection program. The question was raised as to whether the SCC was active or dormant. This paper describes the resultant investigation to determine if a particular service crack was actively growing. The strategy adopted was to assess the appearance of the fracture surface of the service crack and to compare with expectations from laboratory specimens with active SCC. The conclusions from this study are as follows. To judge whether a crack in the service pipe is active or dormant, it is reasonable to compare the very crack tip of the service crack and a fresh crack in a laboratory sample. If the crack tip of the active laboratory sample is similar to that of the service pipe, it means the crack in the service pipe is likely to be active. From the comparison of the crack tip between the service pipe and the laboratory samples, it appears likely that the cracks in the samples extracted from service were most likely to have been active intergranular stress corrosion cracks. (C) 2003 Elsevier Ltd. All rights reserved.

Evaluation of the BEASY program using linear and piecewise linear approaches for the boundary conditions

The boundary element method (BEM) was used to study galvanic corrosion using linear and logarithmic boundary conditions. The linear boundary condition was implemented by using the linear approach and the piecewise linear approach. The logarithmic boundary condition was implemented by the piecewise linear approach. The calculated potential and current density distribution were compared with the prior analytical results. For the linear boundary condition, the BEASY program using the linear approach and the piecewise linear approach gave accurate predictions of the potential and the galvanic current density distributions for varied electrolyte conditions, various film thicknesses, various electrolyte conductivities and various area ratio of anode/cathode. The 50-point piecewise linear method could be used with both linear and logarithmic polarization curves.

Corrosion behaviour of a pressure die cast magnesium alloy

High pressure die casting is the most important production method for casting magnesium alloy components, and uniformity of appearance is an important criterion for acceptance of a component by customers. This paper investigates the influence of uniformity in surface appearance of diecast AZ91D plates on their corrosion behaviour. Through immersion, hydrogen collection and weight loss measurements it was found that corrosion is more likely to occur on the areas of the plate that appear to be darker, leading to a non-uniformly corroded surface. Microstructural analysis showed that the non-uniformity in appearance is related to a difference in the morphology and distribution of porosity across the surface of a diecast AZ91D plate. The darker areas of the surface are high in porosity which breaks the continuity of the beta-phase network and provides shortcut paths for corrosion from the surface to the interior of the casting. The brighter shiny areas of the surface are much less porous, with isolated pores being confined by corrosion resistant beta-precipitates thus reducing the corrosion rate.

Influence of the beta phase on the corrosion performance of anodised coatings on magnesium-aluminium alloys

The effect of the beta phase in Mg-Al alloys on the corrosion performance of an anodised coating was studied. It was found that the corrosion resistance of the anodised coating was closely associated with the corrosion performance of the substrate alloy. In particular, Mg alloys with a dual phase microstructure of alpha + beta with intermediate aluminium contents (namely 5%, 10% and 22% Al) after anodisation had the highest corrosion rate and the worst corrosion resistance provide by the anodised coating. The poor performance of an anodised coating was attributed partly to lower corrosion resistance of the substrate alloy and partly to the higher porosity of the anodised coating. (c) 2004 Elsevier Ltd. All rights reserved.

Patch and whole of surface mass-transfer rate measurements on a stepped rotating cylinder electrode

A recently developed whole of surface electroplating technique was used to obtain mass-transfer rates in the separated flow region of a stepped rotating cylinder electrode. These data are compared with previously reported mass-transfer rates obtained with a patch electrode. It was found that the two methods yield different results, where at lower Reynolds numbers, the mass-transfer rate enhancement was noticeably higher for the whole of the surface electrode than for the patch electrode. The location of the peak mass transfer behind the step, as measured with a patch electrode, was reported to be independent of the Reynolds number in previous studies, whereas the whole of the surface electrode shows a definite Reynolds number dependence. Large eddy simulation results for the recirculating region behind a step are used in this work to show that this difference in behavior is related to the existence of a much thinner fluid layer at the wall for which the velocity is a linear junction of distance from the wall. Consequently, the diffusion layer no longer lies well within a laminar sublayer. It is concluded that the patch electrode responds to the wall shear stress for smooth wall flow as well as for the disturbed flow region behind the step. When the whole of the surface is electro-active, the response is to mass transfer even when this is not a sole function of wall shear stress. The results demonstrate that the choice of the mass-transfer measurement technique in corrosion studies can have a significant effect on the results obtained from empirical data.

A critical review of the stress corrosion cracking (SCC) of magnesium alloys

This review aims to provide a foundation for the safe and effective use of magnesium (Mg) alloys, including practical guidelines for the service use of Mg alloys in the atmosphere and/or in contact with aqueous solutions. This is to provide support for the rapidly increasing use of Mg in industrial applications, particularly in the automobile industry. These guidelines should be firmly based on a critical analysis of our knowledge of SCC based on (1) service experience, (2) laboratory testing and (3) understanding of the mechanism of SCC, as well as based on an understanding of the Mg corrosion mechanism.

Simulation of galvanic corrosion of magnesium coupled to a steel fastener in NaCl solution

The galvanic corrosion of magnesium alloy AZ91D coupled to a steel fastener was studied using a boundary element method (BEM) model and experimental measurements. The BEM model used the measured polarization curves as boundary conditions. The experimental program involved measuring the total corrosion rate as a function of distance from the interface of the magnesium in the form of a sheet containing a mild steel circular insert (5 to 30 mm in diameter). The measured total corrosion rate was interpreted as due to galvanic corrosion plus self corrosion. For a typical case, the self corrosion was estimated typically to be similar to 230 mm/y for an area surrounding the interface and to a distance of about I cm from the interface. Scanning Kelvin Probe Force Microscopy (SKPFM) revealed microgalvanic cells with potential differences of approximately 100 mV across the AZ91D surface. These microgalvanic cells may influence the relative contributions of galvanic and self corrosion to the total corrosion of AZ91D.

Boundary element method predictions of the influence of the electrolyte on the galvanic corrosion of AZ91D coupled to steel

This research investigated the galvanic corrosion of the magnesium alloy AZ91D coupled to steel. The galvanic current distribution was measured in 5% NaCl solution, corrosive water and an auto coolant. The experimental measurements were compared with predictions from a Boundary Element Method (BEM) model. The boundary condition, required as an input into the BEM model, needs to be a polarization curve that accurately reflects the corrosion process. Provided that the polarization curve does reflect steady state, the BEM model is expected to be able to reflect steady state galvanic corrosion.