Avoiding aluminium’s corrosive power

By Andy Holland, technical and marketing manager at Rapierstar.

While we think of aluminium as a material that takes us away from any risk of corrosion or degradation, did you know that it’s ability to react with other metals can be a major cause of early failure of aluminium windows and doors?

Aluminium fenestration may be synonymous with engineered quality and wonderful visual appeal, but this can be undermined because the wrong type of fasteners are being used in fabrication. They may represent only a tiny part of the overall finished product, but fasteners can be the difference between success and failure – it all depends on which type of metal they contain.

That’s why if you are fabricating different materials under one roof – such as PVC and aluminium – you need to take extreme care. Any old screw will certainly not do for aluminium if you want the window or door to last in the long term and deliver on its promise.

Fasteners mainly come in coated carbon steel and several types of stainless steel with different levels of corrosion resistance. But for aluminium windows and doors, the only choice is austenitic stainless steel screws (sometimes referred to as A4 or grade 316).

The reason lies in the science of metallurgy. Think of how a battery works – different metals react with an electrolyte to generate an electric current until the metals eventually corrode to nothing and the battery runs flat. A similar reaction – galvanic corrosion – takes place when certain metals are combined. One metal corrodes preferentially in contact with another, in the presence of an electrolyte. In the case of aluminium windows and doors, aluminium is one metal, the electrolyte is water, and the other metal is whatever your fastener is made from.

The extent of galvanic corrosion depends on the two metals being combined. And in short, inserting carbon steel or martensitic stainless steel into aluminium will result in a far quicker corrosive reaction compared with that of aluminium and austenitic stainless steel.

So austenitic is the only choice, but isn’t its lack of magnetism is going to make fabrication trickier? Not the case when an AC2 driver bit is used in a well-formed screw recess. Available from Rapierstar, this provides a better hold on the screw during fabrication for greater production efficiency.

Using alternative screw types is very risky. A martensitic drill point or a bi-metallic fastener could result in premature failure of the screw when used in aluminium.

Specifiers understand the reason for using austenitic fasteners in aluminium and they will expect them for the premium quality their clients demand. Failure to manufacture to the spec will mean costly consequences sooner than you think. The galvanic process could result in the hardware becoming loose or detached and the window or door no longer functioning as intended. That means expensive call backs, particularly for high rise commercial buildings where safe access and user disruption will need to be managed and budgeted for.

More costly is the long term business damage. Online reviews, word of mouth and law suits can all effect a business that fails to deliver what it promised to its customer. Is it really worth it to save a few pounds?

Using austenitic stainless steel fasteners in aluminium fenestration is the only way to give your clients a credible warranty. In the case of Rapierstar, that’s a 10-year guarantee on the fastener performance.