Put your metal where your mouth is

The most popular materials for use in sign structures are aluminium and steel. But these materials come in many different types and grades, some of which are more suitable than others for use in signs. Also, the decision to use either aluminium or steel is not necessarily as straightforward as it may seem. When requirements such as the need to weld or mix materials are added to the equation, the decision becomes a little more complex.

So, this month I shall examine the relative merits of aluminium and steel, and try to unpick some of the key factors used in specifying the most appropriate material for the job.

Types of steel

Mild steel is really a ‘catch all’ term which could be used to describe a number of different types of steel. For structural purposes in the UK S275J2H or S355J2H are the most common, and put simply the S355 grade is ‘stronger’ than S275. Steel can also be ‘hot’ or ‘cold’ formed—‘hot’ forming being generally ‘stronger’ when talking about sections used for sign structures.

Types of aluminium

Aluminium comes in many different grades of base alloy and temper. How aluminium is ‘alloyed’, or mixed, with other materials (like copper or zinc) and how it is finished will affect its overall strength. For instance, 6063 T6 is a good structural aluminium for use in designs that require a degree of strength in components such a posts that need to resist windload. An aluminium such as 1050 is generally ‘softer’, easier to ‘work’ and more cost effective weight for weight. However, to use this aluminium for the same posts would require a much larger section, if it would work at all; therefore more weight negating any probable cost advantages.

Welding aluminium

Care should also be taken when choosing an aluminium for a structural application, particularly if it is to be welded. In simple terms, welding any aluminium will ‘weaken’ it structurally, but with some aluminium grades it can weaken it very significantly. When aluminium is welded, the heat acts on an area of the material approximately 25mm away from the weld itself, the effect of which can weaken the structural properties of the material. If in any doubt this is a topic that requires professional advice.

Combating corrosion 

When dissimilar metals are in direct contact, one will corrode more than the other through an electrochemical process called ‘galvanic corrosion’.

Untreated aluminium has good corrosion resistance as it naturally forms a thin ‘oxide’ (goes white) layer which prevents further oxidation. This layer is impermeable and if it is penetrated—scratched through to the original metal—will just reform an oxidised layer. Steel, on the other hand, is less able to resist corrosion and will continue to ‘rust’ if left exposed.

Aluminium is normally at greater risk from galvanic corrosion when used with for example steel or stainless steel. This will not occur in dry, indoor conditions. But if this occurs in damp external conditions the aluminium will suffer serious corrosion if left unchecked. To prevent galvanic corrosion occurring, a ‘non conductive’ barrier should be used between all areas where surface-to-surface contact is present. For example, a non-water absorbent nylon bush can be used through holes in aluminium where steel bolts are required.

A balanced view

The relative merits of aluminium versus steel is not just a simple, ‘let’s use aluminium as its lighter and doesn’t rust’ decision. In some circumstances, the aluminium sections required may have to be so large they end up being heavier than the equivalent steel sections needed. Again, if you need to weld aluminium, do not forget that larger sections may need to be used because of the ‘weakening’ effect of this process.  But aluminium is very versatile and mechanical fixings can often be used instead of welding. So, do not automatically dismiss either material; assess probable advantages and disadvantages against the priorities of each design, and it should help provide a safe and cost effective sign.