"Windhover" is a dramatic stainless steel sculpture created by the late Lenton Parr, located on the eastern foreshore of Port Phillip Bay in Sandringham, Melbourne.
Unveiled in December 2001, the sculpture's vertical lines and arcs are evocative of the yachts often seen sailing out on the Bay.
However, two and a half years of zero maintenance and exposure to salt spray from the bay have taken their toll, turning the surface of the stainless steel a blotchy brown.
Called tea staining, it's caused by deposition of salt on the surface which is then trapped in the crevices of the brushed finish.
Regular reactivation by rain has perpetuated a corrosion cycle leading to quite rapid and severe surface staining.
The problem was how to clean the sculpture and then to ensure that it would remain protected from tea staining in the future.
Conventional weld pickling products containing hydrofluoric acid are very aggressive and risk damage to or discolouration of the surface.
Strong acids may also create an environmental and safety hazard when used in such a public place.
Many cleaning formulations are available based on phosphoric, sulphamic, oxalic or nitric acids. They have various degrees of handling and disposal restrictions.
The formulations may also contain mid abrasives and wetting agents/detergents to aid the cleaning process.
In July 2004, ASSDA Member, Revolution Advanced Metals and Materials, used a cleaning paste based on a moderate concentration of phosphoric acid which is relatively safe to handle.
Inadvertent skin contact by this product does not cause the burning and possible ulceration associated with strong concentrations of nitric and hydrofluoric acid preparations.
The cleaning product was brushed on and left to react for 3-4 hours. The brown tea staining gradually disappeared.
In some particularly bad sections a second application was necessary to completely remove all traces of the staining, but it left a completely blemish-free surface.
In this case, residue from the cleaning product was simply washed away with water. In other cases, however, check with local authorities for correct disposal procedures.
One of the problems when washing stainless steel with water is the streaking caused by uneven drying.
This was very noticeable on the sculpture.
Also, because it is unlikely that ongoing regular cleaning will occur, it is also important to limit the access of chlorides to the surface. Otherwise the staining problem will recur.
To overcome both these problems, a water-based protective product with oils and non-ionic surfactants but no phosphates was sprayed on and wiped over.
After polishing with a dry cloth all streaking vanished. It left an invisible film that stopped further streaking and fingermarks.
Best of all, it brought up the lustre of the brushed finish, and left Windhover looking as good as the day it was made.
Regular re-application should maintain the finish and help prevent tea staining in future.
Correct design, fabrication and on-going maintenance will all assist in keeping stainless steel sculptures and other structures erected adjacent to the coast in good condition.
Words and images courtesy of Jim Picot, Revolution Advanced Metals & Materials.
This article featured in Australian Stainless magazine - Issue 29, September 2004.