Essential Metal Knowledge

Is Galvanising Mild Steel Necessary?

You are probably aware that galvanising is a process that stops steel from rusting (corrosion). As with plenty of subjects, the devil is in the detail; there is a lot to be aware of around the subject. Specifically, when do you need to galvanise external architectural fittings, such as planters and benches? What galvanising techniques are there? Is there a cost saving to be made by not galvanising? Your answers and more below:

What is Mild Steel

Mild Steel is the standard base material for all external furniture because its properties are well balanced. However, it is not an alloy which means the composition is almost 100% iron. Of course, iron is highly vulnerable to corrosion which is why mild steel needs protection.

What is Hot-Dip Galvanising?

Hot-dip galvanising is the premier technique when considering rust protection. Throughout this article, all mentions of galvanising will refer to the hot dip technique.

The process is not complex. Take for example the production of a steel planter – the mild steel planter is delivered to the galvanising shop pre-formed (in the shape of the final product). First, the planter goes through a cleaning process. Then the planter is dipped into a molten zinc bath which is at around 450 degrees. The planter is held in this bath until the temperature of the steel matches the temperature of the zinc, usually around 5 minutes.

The chemical reaction between the molten zinc and mild steel creates a coating that is metallurgically bonded to the steel. Without the jargon – the steel is coated with a protective layer that is engrained into the steel and cannot be scraped off. The protective layer provides maintenance free corrosion protection for over 50 years.

Other Galvanising Techniques

There are numerous other galvanizing techniques, but for external furniture the only other technique used is a process known as ‘cold galvanising’ or ‘zinc primer’. This simply involves spray painting the pre-formed component in a zinc-rich paint.

The reason why this is an inferior technique when compared to hot-dip is because the zinc does not become engrained into the steel in the same way because of the presence of binders and other chemicals in the paint. In fact, the hot dip zinc coating has an adhesive strength that is 7 times stronger than a cold galvanised finish. Therefore, cold galvanizing works well until the component is scratched – then the steel is completely open to rust.

When is Galvanising Necessary?

1, Obviously, if there are no other finishes applied to the steel, galvanising is necessary. If not, the steel will have no protection and rust immediately.

2, Galvanising is also necessary if powder coating the mild steel. In theory, the powder coating will form a protective layer and prevent the steel underneath from rusting. However, in practise is not always the case as any small scratch will result in that area succumbing to rust. Using a non-galvanised planter as an example, as it is being installed small scratches are inevitable and the wear and tear of daily use will result in further damage. The result is a planter that looks very worn even if it is only a few years old.

When is Galvanising not Necessary?

1, When using Corten (Weathering) Steel. The top layer of Corten rusts and forms a protective film, protecting the steel underneath. Therefore, Corten should not be galvanised.

2, When using Stainless Steel. Stainless Steel, in particular architectural grades such as 316, contain high chromium content. This means that Stainless Steel is inherently corrosion resistant, eliminating the need for galvanising.

Galvanising Challenges

When galvanising, there are issues to be aware of. To get first-hand knowledge about the most problematic of these, Owen Johnston, a Project Manager at Logic Manufactured Bespoke was interviewed.

Degassing. “Degassing (also known as Pinholing) is the main issue to watch out when powder coating a galvanised component. Basically, tiny gas bubbles trapped in the steel during the bathing process pop through when the component goes through the curing oven at the end on the powder coating process. This creates noticeable little craters across the finished surface. To prevent this, heat the galvanised component pre-powder coating in a oven that is a bit warmer than the powder coating curing oven – this will expunge the gas bubbles to give a smooth powder coated finish.”
White Rust. “A Galvanized finish does not rust. However, if water puddles on a galvanised surface, in some cases the zinc will oxidise causing white chalky stains. It doesn’t affect the structure of the steel, but it is unsightly. Obviously, this isn’t a problem for external planters and seating – powder coating will protect the zinc underneath and the water needs to be continuously standing on the surface for a long time.”
Size Limitations. “Industry standard galv baths are 7m Long, 3.2m Deep and 1.2m Wide. Therefore, it is difficult to galvanise large items. You can double-galv (placing each end in separately to get a full coating) but this leaves unsightly horizontal marks – fine for hidden steel but not if the component is on show.”

Conclusion

In conclusion, it is necessary to galvanise mild steel unless you are using Stainless Steel or Corten. If the galvanizing is done using the hot dip method and potential problems are avoided, the longevity of the external steel will be greatly increased.