The Fundamentals of Corrosion

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CED Working Day and Symposium on Atmospheric Corrosion in Industrial Applications
The tenth CED Working Day was held on Tuesday 24 April 2018 at The Centre, Birchwood Park, Warrington. Some thirty-nine delegates were welcomed by Chairman, Nick Smart. In addition, there were several exhibition stands, a visit to Wood plc test facilities, and CED Working Group Meetings.

Professor Stuart Lyon (University of Manchester) gave an introductory lecture entitled, ‘Introduction to atmospheric corrosion – mechanisms etc’. Given the large surface areas of materials exposed to the atmosphere, annual losses due to corrosion in the UK are in the order of several hundred million pounds. The atmosphere is one of the most common natural environments to which materials are exposed, however unlike most environments, the atmosphere does not provide constant exposure conditions. Thus, the corrosion rates of similar specimens exposed to the atmosphere have poor reproducibility. The main constituents of the atmosphere are essentially constant, however minor components can affect corrosion rates significantly, for example, the concentration of water vapour can vary over a large range. Three primary sources of air pollution are, volcanic action, vegetation and animal wastes, however sea-spray and dust from the earth also need to be considered. Pollutants are also classified as gaseous, particulate or aqueous (dissolved in rain) and man-made atmospheric pollution includes that from burning fossil fuels, chemical processing, sewage treatment and farming. Stuart went on the explain climate effects, macroclimates and microclimates, and that ISO 9223 provides a classification scheme for ranking pollution in a particular climatic location, which is based on the deposition rates of SO2 and Cl-. Atmospheric corrosion only proceeds when sufficient water is present to solvate ions generated during anodic and cathodic reactions. By definition, at 100% Relative Humidity (RH) condensation occurs. Metal corrosion cannot occur unless there is sufficient liquid water on the surface. The time during which RH exceeds a critical value is defined as the ‘Time of Wetness’ (TOW) and ISO 9223 also includes a scheme for ranking a particular climate in terms of TOW (hrs/year). The standard also provides a classification scheme for the ‘Corrosivity’ of an environment, based on the expected annual corrosion rate over 1 year. Finally, specific mechanisms relating to the atmospheric corrosion of iron and zinc were outlined.

Richard Bewell (Engineering Manager, BAM Nuttall) gave a presentation on, ‘Atmospheric corrosion prevention in the windpower industry’. The ‘Blyth Demonstrator Project’ (located about 5.6 km off the North-East coast of England at a water depth of about 38m) has five 8.3 MW turbines, capable of supplying power to 34,000 homes, with a design life of 26 years, and commercial operations commenced in October 2017. The design is essentially a monopole with a heavy reinforced concrete base approx. 30m diameter x 1.1m deep and using over 1,800m3 of concrete with over 500 tonnes of steel reinforcement per foundation. The towers comprised steel shafts, 60m high x 7.5m diameter with a wall thickness of 70mm. For protection in the splash zone and for atmospheric exposure, Norsok C5M specification was applied.

Cristano Padovani (Wood plc) addressed, ‘Modelling the corrosion behaviour of intermediate level radioactive waste (ILW) containers during prolonged exposure to atmospheric conditions’. The stages involved in waste management are, immobilise and containerise, interim storage, transport to an underground geological disposal facility (GDF) and utilise man-made and natural barriers to prevent/minimise release. The atmospheric corrosion of the stainless steel containers, used for medium- and long-term storage , is induced by chloride-containing aerosols, generated by hygroscopic chloride salts on surfaces. Temperature and relative humidity (RH) determine electrolyte characteristics (e.g. concentration) and hence, corrosivity. The factors investigated were, environmental and corrosion monitoring, pit initiation, and mechanistic and SCC initiation studies. Temperature and humidity fluctuate daily and have seasonal variation for above-ground facilities, but are typically more stable underground. It was found that MgCl2 and CaCl2 were inherently much more corrosive than NaCl (especially with regard to SCC), and that high enough RH can lead to dilution of corrosive ions and prevent initiation or cessation, and that SCC initiation was severely inhibited by induced surface compressive stresses. Mechanistic studies in bulk solution indicated that, under polarization, follow kinetics such as, D = A tn, where D=pit depth, A= a constant, t= time and n is an exponent. Furthermore, it was found that SCC propagated very fast after an ‘apparent’ incubation period. A Parametric model – Atmospheric Corrosion of stainless Steel in Stores (ACSIS) was developed, comprising three basic modules, environmental (is the surface wet ?), corrosion initiation (does corrosion initiate ?), and corrosion propation (if so, how much damage results ?). The Laycock-White-Krouse (LWK) mechanistic model was described and used to relate pit depth as a function of time for high RH’s and different chloride surface concentrations.

‘Assessment of corrosion under insulation and engineered composite wraps using pulsed Eddy-current techniques’ was described by Bill Brown (TRAC Oil and Gas Ltd). This presentation touched on the NDT aspect of atmospheric corrosion. Corrosion under insulation (CUI) is possibly the greatest unresolved asset integrity problem in industry. Current methods for measuring wall thickness under insulation, without removing it, all have severe limitations. In total, there are three eddy current devices available for this. However, TRAC evaluated the recently-introduced ‘Lyft’ – a high-performance reinvented pulsed eddy current (PEC) technique) and also Maxwell produced PEC equipment. Although radiography may also be used, it is generally limited to 6 in. diameter pipework, and computer-aided tomography has also been used. A combined PEC and digital radiography technique was developed by Shell in the early 1990’s. A simplified working model of PEC was described and that the concepts of footprint and average area of a probe are key to understanding what a PEC can and cannot detect. The footprint is affected by the size of the probe and the distance from the component or structure being examined from the probe. The footprint is of utmost importance, as it is the decisive factor in determining the dimension of the inspection grid, edge effect and the smallest volume or defect. Nine case histories using this technique were then given.

Professor Geraint Williams (Swansea University) addressed, ‘Preventing corrosion of galvanized steel in the non-chromate age’. The EU has set a ‘sunset’ date of 2019 for the replacement of hexavalent chromium corrosion inhibitor. The problem of how the performance of new inhibitor pigments could be quantified under atmospheric corrosion was raised. Methods included, external weathering, accelerated salt spray and EIS (immersion in corrosive solutions). Assessment of organic coatings carried out at Swansea University included the use of a Scanning Kelvin Probe (SKP). This involves the use of a reference electrode in a test chamber, without physical contact – i.e. not in a ‘bulk’ electrolyte, providing a ‘spatial map’ of corrosion potentials. Ion exchange materials are promising chromium-free anti-corrosion pigments, in which ‘smart release’ inhibitors are only released whenever a corrosive environment is encountered. ‘Hydrotalcite’ is a layered double hydroxide of general formula Mg6Al2CO3(OH)16.4H2O. The carbonate anions that lie between the structured layers are weakly bound giving the material anion exchange capabilities. Inhibition arises by the sequestered aggressive Cl- ions being exchanged for less aggressive ions. The action of this ion exchange mechanism was studied on hot-dip galvanized steel by SKP. The use of benzothiazole inhibitors as a non-chromate alternative was also mentioned. The mechanism is to stifle the underfilm oxygen reduction reaction. Finally, the next generation of Zn-MgAl alloys for galvanizing was introduced. These are very heterogeneous, comprising a three phase material.

John Broomfield receiving the Paul McIntyre Award from Sarah Vasey.

The second annual Paul McIntyre Award was presented at the meeting to John Broomfield, by Sarah Vasey, President of The Institute of Corrosion. This Award is presented to a senior corrosion engineer, over the age of 30, who as well as being a leading practitioner in his field, has advanced European collaboration and international standards development (in keeping with Paul’s area of interest). Recipients must have established an international reputation in the field of corrosion engineering. John had originally a background in spectroscopy and applied these studies to monolayers on steel surfaces. Later, he worked on problems with PWR reactors in the UK, later transferring to Taylor Woodrow and subsequently carried out work on concrete pre-stressed pressure vessels. Some very interesting slides on the pioneering work he was involved with were shown.
At the close, Nick Smart thanked the speakers, the delegates for attending the working day, Wood plc staff for the conducted tours of facilities, and the exhibitors, for a successful and enjoyable event.
Editor’s note: Text copyright: David Nuttall, released under CC-BY-NC.