Technical Presentations at the October 2000 Meeting

Developments in modelling crevice corrosion,
John Oldfield (Cortest).

The mathematical modelling approach to crevice corrosion has been used to develop a test which is capable of predicting the performance of stainless steel components in the marine environment. The test requires a single electrochemical data point as input which can be determined rapidly in the laboratory. The test output, via user friendly software distinguishes between different components e.g. flanged joint, O-ring, compression fitting, etc. and takes account of temperature and chlorination level in its predictions.

The results of this work have been reported in "Prediction of Service Life of Stainless Steel Components in the Marine Environment- A New Rapid and Reliable Crevice Corrosion Test", John W Oldfield and Gary L Masters, Corrosion 97, NACE International, Paper no 429.

Development of corrosion resistant coatings by high velocity oxyfuel (HVOF) spraying
Dave Harvey (TWI).

HVOF spraying has been industrially established in the UK for about 15 years. The high pressure oxyfuel combustion process represents a technically and economically competitive alternative to plasma and detonation flame spraying. It is estimated that about 80 systems are currently in service, primarily for depositing wear resistant tungsten carbide cobalt coatings. In recent years, a number of organisations like TWI have attempted to demonstrate the suitability of HVOF coatings for prevention of corrosion, particularly in saline environments (immersion, splash zone, coastal atmospheres). The main choice of coating materials has focused on nickel alloys such as Inconel 625, Hastelloys C22 and C276. The presentation covers:

• Basic principles of the HVOF process
• Relationships between process parameters, coating microstructure and corrosion resistance
• Graded metal ceramic coatings for combined corrosion and wear resistance
• Developments in thermally sprayed aluminium

Electrochemical techniques for characterising the corrosion susceptibility of welds
Alan Turnbull (NPL).

The effectiveness of electrochemical techniques in characterising the corrosion resistance of welded super-duplex stainless steels is being evaluated with a view to development of a method for in-situ measurement in the field. Localised corrosion of the alloy-depleted zone associated with sigma phase formation is the primary concern, with specific interest in the extent to which propagation of corrosion damage would be sustained due to connectivity of depleted zones. Testing was carried out on a solution-annealed super-duplex stainless steel, on iso-thermally heat-treated samples, and on samples from welded pipe sections produced with variable heat input in order to generate different levels of sigma phase content.

Three electrochemical methods were utilised, viz. critical pitting temperature (CPT) measurement, electrochemical potentiodynamic reactivation (EPR) and a NPL galvanostatic test. The CPT measurements were conducted in accordance with ASTM G150. EPR testing was carried out using a solution of 3M HCl at a scan rate of 1 mVs-1. The ratio of the charge associated with reactivation to the charge of activation was used as an index of susceptibility. The galvanostatic test involved the application of a constant current density of 100 mAcm-2 in an aggressive environment and monitoring the potential response. For a susceptible material the potential will tend to increase but then decrease markedly whilst for a non-susceptible material the potential will remain high.

For samples isothermally heat treated for different periods at the same temperature (800 °C), there was a consistent relationship between volume fraction of sigma phase and the CPT, the EPR charge ratio, and the potential in the galvanostatic test. For welds the situation was more complex. The CPT measurements were not consistent with the heat input but this may be due to positioning of the probe over the weld and only partially on adjacent plate. These measurements are being repeated with the probe directly on the zone of sigma-phase precipitation. The EPR measurements were made with the probe over the weld and also, in separate experiments, over the sigma-phase precipitation zone. The charge ratios in both cases were very low compared with the isothermally heat treated samples. More importantly, the values measured over the welds were somewhat larger than over the sigma-phase precipitate region. The EPR method may be less effective for application to inhomogenous materials and would appear not to be ideal for reliable discrimination of the extent of sigma-phase depletion. The NPL galvanostatic method has not yet being applied to the welds as yet but work is in progress.

Of the three electrochemical techniques, the galvanostatic test shows most promise for field application but further evaluation is required.