Case Studies - Aviation

Interaction of lubricants with magnesium alloy components in aircraft turbine engines.

Over the past 10 years, a number of single- engine jet aircraft have experienced sudden oil filter blocking, often only tens of hours after the maintenance period and cleaning of the filter. The dark purple tacky deposits found inside resulted in 80 per cent filter blockage even though the lubricants used were approved to Def Stan 91-100.

QinetiQ was requested to identify the deposit, find the cause and mechanisms involved and, if possible, come up with solutions.

Stripping down an engine, engineers found that magnesium alloy components were coated for corrosion protection, initially with a chromated layer, then an epoxy coat or lacquer. However, these magnesium alloy components showed signs of pitting and corrosion on the edges of flanges where no lacquer finish was visible.

Analysis of the deposits indicated that a reaction was taking place between these inadequately protected parts of the magnesium casing and phosphorus based additives used in the lubricants.

We therefore carried out corrosion tests with various mixtures and magnesium alloy. The worst case was considered and initial tests used unprotected alloy with the chromate protection removed. No effect on the magnesium test panels was observed with the neat lubricant or neat compressor wash fluid.

With the mixtures of lubricant and water, however, significant corrosion was seen. Corrosion products were similar to the filter deposit, containing magnesium and phosphorus. The corrosive effect was found to occur on chromated surfaces of magnesium alloy as well as on completely unprotected surfaces.

From these studies we were able to build up an idea of the mechanisms involved, and how a sudden increase in engine power such as at take-off might cause rapid deposition of deposits onto the filter.

In order to assess which lubricants have less deleterious effects on the magnesium alloy components, we devised a test to rank aviation lubricants in terms of their propensity to cause corrosion on magnesium alloy test panels.

We were then able to make a recommendation to the operator on a change of oil grade that would greatly reduce the rate at which the deposits are formed.