A fix for corrosion’s hidden dangers

// the building envelope

Specifying the right fasteners for doors and windows is crucially important to ensure that they stand the test of time over a long, low maintenance service life. Seeking the right technical advice on fasteners is, therefore, the key to optimising value and tenant satisfaction.

In the pursuit of providing windows and doors which offer longevity, enhanced security, optimum safety and robust warranties over a long, low maintenance service life, the key considerations are likely to be frame’s design, the glass, locking system, handles and hinges. But what about the quality of the components that hold windows and doors together – the fasteners?

It is often assumed that the specification of the window and door fasteners only really needs careful attention where there is a high risk of corrosion, and that it is an issue confined to coastal areas where the air is particularly aggressive to metals. This is certainly not the case – corrosion can be a problem in any location.
Specifying the wrong types of fasteners can lead to premature failure of the window or door. These smallest of components with a tiny unit cost can, therefore, easy lead to extremely costly remedial work and avoidable tenant disruption and inconvenience.

Consider too that fastener failure could also result in a window or door becoming unsafe when subjected to an impact load, or structurally weaker so that it no longer resists a burglary attempt. The implications, of course, could be significant with all the best safety and security intentions being undermined and opening the door to tenant claims and litigation.

Typical causes of corrosion

When most of us think of corrosion we probably think of the typical highly visible iron oxide hydrate commonly referred to as red rust – but corrosion can also be invisible. Corrosion does not need a wet or salt laden atmosphere to occur. A relative humidity of just 60% is all it needs, and the chances of corrosion occurring increase in the presence of common airborne pollutants and cleaning agents.

Equally corrosion does not always show on the surface of a corroded item. Some types of corrosion, such as stress corrosion cracking, develop unseen within the structure of a fastener or other component. In these cases, the first time the issue becomes apparent is when something breaks. This could be catastrophic for an installed door or window, with significant safety and security risks for building occupants.

The principal causes of fastener corrosion in windows and doors are uniform corrosion and crevice corrosion, both of which are caused by the accumulation of moisture triggering an anodic corrosion reaction. Protection from uniform corrosion is best achieved by selecting inherently corrosion resistant fasteners or ones with a plating or coating suited to the environment and application.

Crevice corrosion and uniform corrosion share the same anodic corrosion mechanism, and therefore share the same protective measures. The risk of crevice corrosion can be further reduced by eliminating the use of washers, and substituting these for flanged fasteners instead, and making all fastener-to-joint interfaces as smooth as possible.

Another type of corrosion is galvanic corrosion. This is caused by reactions occurring between fasteners and the components they are jointing, where the two metals have differing electromechanical potentials and moisture is present. Joining metals of differing electromechanical potentials causes an electrical current to flow from the less noble anodic metal to the more noble cathodic metal, dissolving the less noble metal in the process.

Mitigating the risks

Fastener corrosion can’t be stopped, it’s an unrelenting enemy. But it can be managed to deliver on a service life promise. The key is to understand the differences between the types of metals that are used to manufacture fasteners for fenestration and how these cope with the different corrosive processes.

The main steel types utilised for fasteners and fixings are:
• Coated carbon steel
• Austenitic stainless steel
• Martensitic stainless steel
• Bi-metallic

Coated carbon steel is the baseline fastener metal – electroplated zinc, plus a clear or yellow passivation layer. Fasteners of this type will need to show evidence of being salt spray tested, showing no corrosion, to 240 hours in accordance with BS EN 9227: 2012. This provides a service condition of Grade 4 to BS EN 1670: 2007.

Coated carbon steel fasteners are ideal for certain applications, being easily capable of providing a window or door service life of at least 10 years. But, the conditions in which coated carbon steel fasteners perform well are not easy to predict, and there are two important reasons why a higher performing product may be sought.

Firstly, location. Many locations will have a harsh environment, where the local atmosphere contains the elements that have a corrosive effect on metal hardware. By this, we don’t just mean coastal regions or industrial areas. Coated carbon steel screws are unlikely to be able to equal the service life of the system into which they are integrated in large parts of the UK because we are never far from water – being an island – and our air is often heavily polluted with corrosive elements.

Secondly, an extended service life for the window or door of up to 25 years may be sought. PVC-U and aluminium frames are easily capable of retaining their integrity for this kind of period, so there is a powerful case for specifying all stainless steel metal components in order to match this.

In both cases, the fastener specification will need to be stainless steel. Best practice for friction stays, for example, is to specify an austenitic grade of stainless steel salt spray tested to 1000 hours giving corrosion protection of Grade 5 under BS EN 1670: 2007.

Austenitic stainless steel must be the fastener metal specified with aluminium profiles due to the particularly aggressive galvanic reaction that can take place between carbon steel and aluminium. This can result in rapid corrosion and failure of the window or door which would lead to costly early repairs.

For PVC windows and doors, austenitic stainless steel is not the only option for countering corrosion.

Fasteners manufactured from enhanced martensitic stainless steel could also deliver PVC windows or doors with the performance that matches the client’s specification. Martensitic is a hardened type of stainless steel, so fasteners of this type have the advantage of being suitable to self-drill into PVC window steel reinforcement – unlike austenitic which is too soft. In certain circumstances, martensitic fasteners are ideal but, as with all fastener options, it is important to seek technical advice before choosing a particular route.

One additional material consideration is a bi-metallic fastener which combines the benefits of an austenitic stainless steel shank with a hardened carbon steel drill point. This overcomes the issue of drilling into steel reinforcement, but bi-metallics are significantly more expensive. They may be the best option, but it is definitely worth assessing all the different materials before choosing bi-metallics.

Making the right choice

Ultimately, corrosion is a relentless challenge to overcome and all too often it is the cause of premature window or door failure. Getting the fastener specification right is vital to achieve client satisfaction and avoid the risk of complaints and reputational damage due to an oversight on a relatively tiny but essential component.

Find out more or access Rapierstar technical advice at www.rapierstar.com

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