Many posts on many sites claim that the corrosion of the spiders is due to galvanic action. I do not agree, I believe it is primarily chemical corrosion.
Should the corrosion have been galvanic between the stainless steel drum and the aluminium spider the majority of the corrosion would have been at the junction of the two metals i.e. at the ends of the arms. I have seen no photographs of spiders corroded in such a manner, nor read of any similar descriptions.
Aluminium is corroded when immersed in an aqueous solution with a pH value above about 8.0. All detergents have to be above about 8.0 or they would not work. The Material Safety Data Sheets put out by Proctor and Gamble state that the pH for one of the liquid ‘Tides’ is 8.0 and for one of the ‘Tide’ powdered detergents as 11.0. Bleach, (sodium hypochlorite) is also very corrosive to aluminium. I should add that for corrosion of the spider to take place these levels are considerably above the levels found in a washing machine during the wash/rinse phases of the cycle.
Sodium carbonate and sodium percarbonate found in some laundry aids are also corrosive to aluminium, provided the required concentrations are reached.
I believe the mechanics of the corrosion are as follows.
Even after the fastest spin small quantities of water will remain on the shaft and towards the centre of the spider. Any recesses in the spider close to the centre will aggravate this situation. This water will contain very, very small quantities of laundry aids used, soil from the laundry, chemicals from the ‘tap’ water, and the results of the interactions of all of the above. Should this water be allowed to stand the water will evaporate until such time as sufficient has gone to allow the pH of the remaining mixture to rise above the threshold at which corrosion will occur.
Additionally the retained water will quickly become foul smelling leading to, I believe, many of the complaints about mold and mildew.
The principal product of the corrosion of the aluminium ally spider is aluminium oxide. This substance is very hard and very abrasive; it is in fact the ‘grit’ found in sandpaper. This aluminium oxide is only very sparingly soluble in water and will adhere very strongly to the donor metal, in this case the spider. As the deposit of aluminium oxide grows on the spider some of this oxide will be carried in suspension in the water and will during the ‘spinning’ phases of the cycle, be deposited on the outer drum. Pictures of this can be seen in the thread here:
Discuss-o-Mat Thread: Everyone meet Horsey->LG
This aluminium oxide when mixed with water will, in my opinion, form a very effective lapping/grinding compound that will, in very short order, destroy the seal allowing water into the bearings. Corrosion and failure of the bearings will follow not to mention the damage also caused to the bearings by the abrasive aluminium oxide. This is not the only cause of bearing failure as there are a few instances recorded ‘on the web’ where the posters have indicated that the bearings have failed but that the seal has not. In fact I can personally vouch for one such case.
There is an informative paper on galvanic corrosion, which also explains why that although stainless steel is more noble than aluminium in this particular instance corrosion does not take place, by a couple of guys from the University of New Brunswick.
Similarly there is an informative one page paper on chemical corrosion, although the author refers to it as ‘micro-galvanic corrosion’ and I grew up calling it ‘pitting corrosion’.
I am sorry but the poster who advocated anodizing the spider to prevent corrosion is in error as explained in the above paper. Anodising is the artificial thickening of the naturally occurring oxide coating.
The ‘tub kit’ referenced in this thread was still available in March of this year, and in the same quote informed me that the inner tub and spider alone was available too! I have previous fitted one of these 'tub kits' to a machine owned by a relative.
I went ahead and replaced the bearings and seal while I was in there. The machine sounds quieter than the day I first got it ! Thanks again to everyone!