Galvanisation: Why it matters for steel garden retaining walls

Is galvanising mild steel necessary?  This article will discuss the factors to consider, particularly those related to external powder-coated garden edging.

Understanding Galvanising for External Architecture

When dealing with steel in outdoor architectural projects, it’s crucial to grasp the ins and outs of galvanising – a process that prevents steel from rusting. In this article, we’ll explore when it is necessary, the various techniques, and whether it’s cost-effective to skip this step.

What is the base material?

Manufacturers normally use mild steel as the base material for galvanised products. This highly versatile material finds application in various industries worldwide, including garden designs. However, you should note that mild steel, composed almost entirely of iron, becomes susceptible to corrosion when exposed to moisture. Therefore, it requires protection.

What is the difference: Hot vs. Cold Galvanizing?

Galvanizing comes in two main methods: ‘hot-dip’ and ‘cold’ galvanizing.

Hot-Dip Galvanizing immerses raw steel in molten zinc at around 450°C, leading to a metallurgically bonded zinc coating. This protective layer acts sacrificially, extending the steel’s durability. It offers robust protection for over 50 years.

Manufacturers usually perform hot-dip galvanizing after fabrication because if done after, the galvanised surface has to ground back before you can weld. When galvanizing fully fabricated items, be aware that welds can expand and that limitations apply to the size and thickness of steel used. For example, hot-dipping 2mm thick steel can cause it to warp and twist due to the heat. Additionally, the size of the galvanizing bath limits how large the steel pieces can be.

Cold Galvanizing is used when thinner sheet steel is to be used. It is slightly less durable than hot dip galvanising so is not used in its uncoated form as is possible with hot dip. Cold galvanised steel is used for powder-coated planters, it offers great protection and avoids corrosion at any damage in the powder coating. Powder-coated sheet steel ensures a smooth consistent powder-coated finish. You cannot weld cold-galvanized sheets because they release toxic fumes.

When is it Necessary?

Galvanising is necessary in all cases when favouring a PPC Finish:

1. Raw mild steel exposed to moisture in the atmosphere will quickly oxidize and rust. Galvanising preserves its life.
2. Even if you plan to powder coat steel, galvanising is essential. While powder coating can provide a protective layer, any scratches or damage to the top coat can expose the steel to moisture and lead to rust. Galvanising ensures added protection.

When is galvanising  Not Necessary?

1. When using Corten (Weathering) Steel, as it forms a protective layer through oxidation.
2. When using Stainless Steel, particularly architectural grades like 316, which are inherently corrosion-resistant, eliminating the need for galvanising.

Conclusion

In conclusion, some form of Galvanisation is essential for mild steel in outdoor architectural applications unless using Stainless Steel or Corten. You can use hot dip in its raw, unfinished form because it offers the greatest amount of protection to mild steel.  Cold galv sheet steel is the best method of achieving a galvanised finish on a steel planter.