Technical Presentations at the January 2003 Meeting
12.1 ‘Coatings to protect nickel aluminium bronze’, Robin Oakley (QinetiQ)
Nickel aluminium bronze is an important alloy for marine engineering, especially in vessels of the Royal Navy. While its performance is generally satisfactory, situations arise which lead to poor performance from the standard alloy, and so to improve the corrosion and erosion resistance of nickel aluminium bronze components it is proposed that coatings may be beneficial. A test program has been established to identify suitable coatings currently available and determine their resistance to cavitation erosion and long-term seawater immersion, and to establish the electrochemical properties and adhesion performance of the coatings and the effect of surface preparation on the fatigue properties of the substrate.
The first coatings to be evaluated were two epoxy -based formulations, with additions of either ceramic particles or low surface energy waxes, and two thermally sprayed ceramic coatings, one applied using Air Plasma Spraying (APS) and the other by High Velocity Oxy-Fuel (HVOF). Control samples of un-coated nickel aluminium bronze were also tested in parallel with the coatings. Samples of nickel aluminium bronze with both machined and as-cast surfaces were examined, to replicate the application of coatings to both flange faces and propellers, and cast pump bodies etc.
Initial results indicate good adhesion and corrosion resistance performance by the epoxy coatings, but that the metallic bond coat used to ensure adhesion of the ceramic coatings to the substrate may be susceptible to corrosion. None of the coatings have proven to offer superior cavitation erosion resistance compared to the un-coated nickel aluminium bronze in the high intensity conditions used for testing, although they would reduce the incidence of high velocity corrosion/impingement attack. [Copyright of QinetiQ Ltd 2002]
12.3 Why Listen to
Electrochemical Noise? Erosion-corrosion
related noise of HVOF aluminium bronze and nickel aluminium bronze
coatings’
T.S. Tan, R.J.K. Wood and J.A. Wharton
(Surface Engineering & Tribology Group, School of Engineering
Sciences, University of Southampton)
Erosion and corrosion attack is a common phenomenon in engineering components subjected to marine environments. These attacks are jointly caused by seawater and sand particles that are entrained in it. In order to minimise material wastage from the above mechanisms, aluminium bronze alloys have been used for fabricating marine components. Under service conditions, erosion and corrosion often occur at localised areas, hence it is both expensive and time consuming to fabricate a new components for replacement. The HVOF coating technology allows in-situ repair of an eroded/corroded component, back into its original geometry, reducing the overall operating cost. Furthermore, it can also be used as a barrier for protecting the substrates against erosion and corrosion.
The electrochemical noise technique has been used extensively for corrosion monitoring. Unlike conventional corrosion monitoring techniques, it is a non-destructive method and does not perturb the system undergoing corrosion. Real time electrochemical data can be obtained from marine components in service and the corrosion behaviours understood.
The objective of this work is to elucidate the degradation rate and mechanisms of HVOF aluminium bronze and nickel aluminium bronze coatings on carbon steel substrates. The coatings were subjected to 3.5% NaCl slurry jet impingement for 5 hours and 5 days, with the latter being carried out to determine the mechanisms of coating failure. Jet velocities in the range of 3.0 to 6.7 ms-1 have been used to investigate the effects of kinetic energy on erosion-corrosion. The corrosion, erosion and erosion-corrosion rate was quantified by gravimetric methods, the flow corrosion rates of the coatings are found to be similar to cast nickel aluminium bronze alloys. Synergism calculations were also carried out, revealing that positive synergism occurs only at velocities beyond 5.0 ms-1. SEM and electrochemical noise analysis showed that porosity within the HVOF coatings is detrimental to its corrosion resistance. Long exposure under flow corrosion conditions revealed transients in the potential noise, indicating corrosion within the pores and at the coating/substrate interface. Electrochemical potential noise is also proven to be useful in detecting coating perforation under erosion-corrosion conditions, revealing a mixed potential between the coating and its substrate. The potential noise standard deviation was also evaluated. Under erosion-corrosion, its values are 10 times higher than that of flow corrosion, reflecting the breakdown of the protective film on the coating surface by impinging sand particles.
12.4 ‘Current Requirements and Developments for
Marine Anti-fouling Paints’,
Julian Hunter, Akzo Nobel International Paints)
Resumé: Recent
IMO legislation has ruled that tributyl-tin (TBT) containing paints are
banned as from 1st Jan 2003 and that they should not be present
on any ship’s surface by 2008. So
far, 7 countries have signed up, of the 25 which are required to make the
legislation effective. These
paints are very effective against the 200 or so species of marine
organisms which cause biofouling as they dissolve away in a controlled
manner, allowing the coating on the vessel to remain smooth and free from
fouling.
Much
effort has been focussed on developing coatings which are as effective as
TBT – so far none has been able to match its performance.
The mechanism whereby TBT is so effective is presented and several
possible compounds are suggested which attempt to mimic its behaviour.
Self-polishing polymer coatings are the latest type to be
developed, and examples are given. These offer a solution which is seen to offer the best
alternative to TBT coatings to date.
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