MERI (Materials and Engineering Research Institute) has been a centre for research excellence in materials since 1990. This excellence was recognised in 2001 when the Research Assessment Exercise (RAE) awarded a 5 rating to the research carried out at MRI. This makes MERI the highest rated department of its type in the new university sector and rated alongside the materials departments of universities such as Liverpool and Queen Mary's.
The major aim of the corrosion group is to elucidate and quantify the processes that contribute to the deterioration and failure of engineering materials through the interaction of material composition, operating environment and the applied/residual stress system.
The Corrosion Group was originally established in 1988. Since this time the group has undertaken a wide range of studies included Environment Assisted Cracking (including corrosion fatigue, stress corrosion cracking and hydrogen embrittlement), Localised Corrosion, Evaluation of Metallic and Non-Metallic Coatings.
The group has access to a wide range of test facilities including Electrochemical Scanning Probe Techniques to assess localised corrosion and more recently an Environmental Scanning Electron Microscope (ESEM), capable of imaging at magnifications typical of a standard SEM, but within environments up to 100% relative humidity.
Corrosion Testing Facilities
Test facilities are available within the Centre for Corrosion Technology for monitoring and quantifying the corrosion degradation of materials and coatings. The Centre for Corrosion Technology has the equipment and expertise to carry out both accelerated environmental and electrochemical tests, as appropriate.
In addition to established electrochemical techniques, the Centre for Corrosion Technology has a suite of instruments for monitoring localised corrosion activity. The aforementioned tests can also be combined with microscopy, which can be a useful tool for assessing corrosion degradation.
Accelerated Environmental Corrosion Tests
Figure 1. Centre for Corrosion Technology
The Centre for Corrosion Technology has facilities available for humidity testing and neutral salt spray testing to ASTM Standard B117. The duration of testing is dependent on the samples and customer requirements and cyclic tests can also be specified, where samples are subjected to alternate wet and dry cycles. The tests are widely recognised in industry and can provide qualitative information regarding the performance of samples or can be used to rank the relative performance of a series of specimens.
Corrosion Testing by Impedance Spectroscopy
Impedance spectroscopy is a powerful electrochemical technique which not only provides quantitative data regarding the corrosion of a sample but can also be used to identify the electrochemical processes occurring. Tests can be performed in different corrosive environments to simulate service conditions. Typically, however, the sample might be immersed in a sodium chloride solution. Impedance spectra can be acquired after short exposure times to obtain information on the corrosion resistance of a material or coating. Alternatively, tests can be repeated over a longer period of time to evaluate the deterioration (increasing corrosion rate) of the sample.
An Impedance provides information not only on corrosion but also corrosion mechanism. Hence the technique can potentially differentiate corrosion due to pitting, general corrosion or diffusion through a polymer film.
D.C Electrochemical Techniques
Figure 2. D.C Electrochemical Corrosion Testing Equipment
D.C Electrochemical Corrosion Testing techniques, include current or potential controlled polarisation tests, cyclitic coltammetry, chronoamperometry and chronopotentiometry. However an effective test method for determining a quantitive corrosion rate value is linear polarisation resistance (LPR). As with all the D.C. electrochemical techniques, the test conditions can be tailored to represent the desired service environment.
Localised Corrosion Test Techniques
Figure 3. Uniscan Model 370 Scanning Electrochemical Workstation
Within the Centre for Corrosion Technology, facilities exist for conducting localised corrosion tests. These techniques have allowed researchers to examine specific phenomena related to corrosion degradation, such as organic coating delamination, stress corrosion cracking, pitting corrosion and repassivation. The range of equipment available includes a scanning vibrating electrode (SVET), scanning Kelvin probe (SKP) and scanning droplet cell (SDC).
The SVET is used to measure the corrosion of conducting materials immersed in an electrolyte. The surface of the corroding material consists of anodic and cathodic sites, resulting in potential gradients forming in the solution. These are measured by a microelectrode which is vibrated perpendicular to the sample surface, hence the name given to this technique. A simple calibration procedure can also allow the measured potentials to be converted to corrosion currents.
The SKP is a non contact technique which, unlike other electrochemical methods, does not require the specimen to be immersed in or exposed to a corrosive medium. Non-conducting materials are essentially transparent to this technique so corrosion beneath paint coatings can ne characterised using this method.
The scanning droplet cell is not a technique, but is a piece of equipment that allows a full range of electrochemical tests to be performed within a small droplet of electrolyte, typically around 1 mm diameter. The electrolyte is constantly replenished, the solution being fed through a capillary arrangement, whereby the spent electrolyte is also removed.
Microscopy can give useful information regarding the extent or mode of corrosion degradation or failure. The range of corrosion testing techniques is therefore supported by electron, optical and atomic force microscopy facilities.
The electron microscopy unit incorporates both transmission and scanning electron microscopes, along with associated sample preparation facilities. MERI's Environmental Scanning Electron Microscope (ESEM) allows non-conducting samples, such as paint coatings, to be examined without the need to render them conductive.
Figure 4. Scanning electron microscope (SEM) image showing corrosion products on hot-dip galvanized steel after immersion in sodium chloride solution
The range of optical microscopes has recently been augmented by the acquisition of an infinite focus microscope (IFM). A frequent problem with optical microscopy is that the depth of field available is not sufficient to image samples with an irregular surface topography. The IFM is able to overcome this limitation by taking a series of images between two focal limits. Computer software then reconstructs a complete image from the individual ‘slices’. In addition to imaging, parameters such as surface roughness and value of voids can also be quantified.
Scanning Electrochemical Probe Techniques (SRET)
The evaluation of corrosion damage remains one largely based upon conventional "macroscopic" test methods i.e. weight loss, polarisation methods etc. These methods assume that the material is corroding "uniformly" over the whole surface area. Where the anodic and cathodic sites of a corrosion cell are fixed on a surface, localised corrosion ensues which can lead to material failure, often in a very short time span.
Recognition of this phenomenon has resulted in numerous attempts to spatially characterise corrosion activity. This in turn has lead to the development of Electrochemical Scanning Techniques, where a surface is scanned in the x and y axis producing a visual 2-D corrosion activity map. The CCT's SRET equipment can be used to evaluate for example breakdown in coatings, weld defects, galvanic and crevice corrosion.
Defect Modelling and Predictive Analysis
The principal aims of the research studies undertaken in the area of EAF are:
· Characterise and understand the mechanisms of failure
· Derive models from which lifetime predictions for EAF can be made
Corrosion fatigue studies of 'defect-free' smooth specimens have shown that pitting corrosion plays a major role in the initial stages of damage, see figures below.
Figure 5. Typical pitting and corrosion fatigue, short-crack development of a high strength steel in 3% NaCl solution.
(1) Torsional Loading. (2) Uniaxial Push-Pull Loading.
(A) = Pitting. (B) = Tensile Crack Growth.
Microstructural fracture mechanics models (see publications) have been constructed to account for the major damage mechanisms, namely pitting, environment-assisted short fatigue crack growth and environment-assisted long fatigue crack growth. In addition these models have been adapted to account for the effects of multiaxial loading under corrosion fatigue conditions.
Mechanisms of Environment-Assisted Failure (EAF)
It is well established that the synergistic effects of a cyclic stress and an aggressive environment reduce the fatigue strength of ferrous and non-ferrous materials. This combination of corrosion and mechanical damage can result in the total elimination of the fatigue limit, particularly for high strength materials, see figure opposite.
Current research has shown that the resistance to localised corrosion combined with the resistance to shear-to-tensile crack formation, dominate, almost exclusively, the process of corrosion fatigue damage; hence the need to understand the influence of chemical effects on microstructural fracture mechanics. For high strength materials where threshold crack sizes are small (i.e., < 100 microns) these initial stages often control the lifetime of engineering components and structures.