What’s The Difference Between Pre-Gal And Hot-Dip Galvanizing?

There are two types of galvanising – ‘batch hot-dip galvanizing’ and ‘pre-gal.’ Batch hot-dip galvanizing is generally referred to as hot-dip galvanising or simply HDG.

What is pre-gal?

Pre-gal, also known as ‘in-line’, ‘continuous’ or ‘mill’ galvanizing, is the galvanizing of steel products such as sheets using an automated process before they have been cut to size and processed. Examples of this includes products such as Duragal (formerly known as SupaGal) by One Steel.

What is hot-dip galvanizing?

Hot-dip galvanizing involves immersing completed steelwork in a bath of molten zinc.

Action Steel hay shed

At Action Steel we hot-dip galvanize both the columns and trusses on every one of our sheds, from hay sheds (like the one above) to shearing sheds. This means they are manufactured in our workshop before being coated and protected from corrosion by being fully immersed in the bath of molten zinc.

Why is hot-dip galvanizing better?

While pre-gal has it’s uses, it does not withstand the corrosive conditions in high-moisture environments such as in a dairy or a hay shed.

So, why else do we hot-dip galvanize our sheds?

1. Achieves a uniform and complete coverage in two ways:

While a hot-dip coating is slightly thicker on corners and narrow edges, coatings such as pre-gal tend to thin out in these critical spots. This is because “either an air knife or mechanical wiper is used to remove excess zinc.”

By comparison pre-gal products are coated in accordance to AS/NZS 4792 and “these products are not normally used in corrosive environments beyond C3 without a paint topcoat due to limited durability.” Because the pre-gal product is cut, welded, punched and drilled after being coated, bare edges are exposed to corrosion.

This is why pre-gal product should be avoided in dairies, hay sheds, yard covers and any sheds in coastal areas.

Action steel shed with hot-dip galvanized trusses

2. Abrasion resistant. There are four layers that form on a hot-dip galvanized article “three of which are alloys of zinc and iron”

  1. The Gamma alloy layer
  2. The Delta alloy layer
  3. The Zeta alloy layer

These alloy layers end up harder than the base steel and rather than just being a surface coating, metallurgically bond to the steel, “becoming an integral part of the steel” itself.

The fourth layer is the pure zinc outer layer which is the only layer that is softer than the steel. The combination of these four layers means the coating will stand up to abrasion and force during transport, construction and erection.

On the other hand, the 100% pure zinc coating of a pre-gal product has minimal alloy layers meaning it can be susceptible to abrasive damage.

3. Cathodic protection. As we outline in our guide, hot-dip galvanizing protects steel in three main ways, but perhaps the most notable way is by acting as a sacrificial coating;

“Because zinc is anodic to steel, even if the durable intermetallic layers of the hot-dip galvanized coating are damaged … adjacent zinc will sacrificially protect the exposed steel.”

4. Coating thickness. One of the three main ways hot-dip galvanizing protects steel is ‘barrier protection.’  Barrier protection occurs due to the simple fact that zinc metal is so dense that no electrolytes such as rainwater can penetrate the coating, meaning the base steel will not corrode.

As shown in the graph below from Galvanizers Association of Australia, the typical coating thickness of hot dip galvanizing is significantly more than that of Pre-Gal (in-line galvanizing) and “in most cases the thickness of the coating is proportional to the durability.”

Steel coating thickness

It is worth noting that “in-line galvanized articles always produce thinner coatings than batch hot dip galvanizing for the same steel thickness and therefore offer less corrosion protection when exposed to the same environment.”

To learn more about hot-dip galvanizing, we recommend reading our ultimate guide to hot-dip galvanizing and checking out our video below on the topic.

 

 

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