Technical Presentations at the January 2001 Meeting

Erosion-corrosion of marine pipework: Failure of HVOF coatings, Andrew Speyer (University of Southampton).

Naval ships contain seawater-carrying pipework for such purposes as machinery cooling and firefighting. This pipework must withstand seawater corrosion and, also, in shallow water erosion by entrained sand. The alloys currently used for this duty do not resist erosion well and are expensive. There is therefore an interest in new pipework materials to address these problems. This presentation represents a progress report on tests on HVOF spayed metal on mild steel coatings, carried out on behalf of DERA.

Satisfactory performance requires coatings of suitably cheap metal, which are hard/strong enough to withstand erosion and are anodic (or at least not cathodic) to the steel substrate. Unfortunately, suitable anodic metals (aluminium and zinc) are also soft. The approach taken here has been to HVOF spray aluminium on steel and to try to harden it by alloying (12% silicon) or by composite formation (12% silicon plus added alumina). Coated samples of these materials have been made by TWI Ltd and tested at the University of Southampton.

Erosion tests (sand in tap water) show that aluminium performs poorly, as expected. Alloying gives improved performance, but addition of alumina does not – traced to excess porosity in the composite. Failure in all cases appears to be entirely ductile, based on lip extrusion and fracture. Further hardening/strengthening, perhaps by use of hypereutectic aluminium/silicon alloy would appear to be indicated.

Static corrosion tests have been carried out in 3.5% NaCl, using Electrochemical Noise Analysis as a monitoring procedure. The steel substrate produces a white noise response which does not change with time. Aluminium based materials produce a power spectral density plot which is negatively sloped. This negative slope reduces with time, suggesting that corrosion on these samples becomes less localised with time. This is rationalised in terms of the progressive blocking of coating porosity by corrosion product, identified as hydrous aluminium oxide. As with the erosion tests, porosity reduction is desirable.

Future work, in a specially constructed erosion-corrosion rig, will concentrate on identifying synergies between erosion and corrosion in this system, and identifying the electrochemical signatures of coating failure.

Corrosion control: the practical application of polymer technology in the marine industry, Gavin Bowers (Belzona).

Polymer systems, in particular epoxy resin technology, have developed significantly since the first leaking pipes were repaired sixty years ago. This presentation outlined the main problems faced by operators where erosion and corrosion occurs, and gave four case histories. For each case history the problem was diagnosed, together proposed solutions and the practical considerations of using polymer technology.

Erosion Corrosion in pumps - isolation of bimetallic components such as bushes using epoxy resins without the inherent problems of welding cast metals.

Corrosion damage to rudder bush housings as a result of the use of interference fit methods in installation. Installation using injection methods were demonstrated.

Coating development for high temperature corrosion in offshore separation equipment - including typical problems associated with material choice for equipment and coatings.

The use of suitable coatings which isolated active metals from their environment is the only method to ensure their protection. It is essential however that the properties of the coating proposed are understood before it is put service to ensure its success.

The Impact of extended docking intervals on the maintenance of ship coating beneath the waterline, David Jones (UMC International plc).

The talk commenced by recording the events, triggered by the Yon Kippur war of 1973, which fundamentally altered the way in which bulk shipping operated. An acute shortage of oil tanker tonnage and dry dock capacity forced a relaxation of Classification Society rules to allow in-water surveys and for the first time, tankers and bulk carriers were permitted to stay afloat for up to 5 years. At the same time, the marine paint industry introduced tributyl-tin (TBT) self-polishing copolymer antifoulings, which provided protection from marine fouling for the full five-year period. However, TBT based antifouling paints are set to be banned on environmental grounds as from 1 Jan. 2003 and the speaker provided evidence to show that the tin-free alternatives only provide protection for 2-3 years.

The speaker went on to explain the power penalty arising from a rough hull and the impact on operating costs associated with marine fouling. Recent non-toxic methods of preventing fouling and in particular, the new 'fouling release' coatings, were reviewed. The relative merits of first and second-generation tin-free paints were considered, together with the cost of providing five years protection using a combination of antifouling paint and underwater cleaning.

The talk continued by discussing developments aimed at automating underwater hull cleaning and concluded by describing a novel method whereby divers can successfully apply paints such as wet-deck epoxies, underwater.