MYTHS TO DISPEL: posts with hot-dip galvanisation after processing are better than pre-galvanised posts


When it comes to support posts in vineyards, the choice of material is now almost exclusively oriented towards metal.

In fact, this product has largely replaced the old wooden posts (except for some protected lands) which are beautiful but not very long-lasting, difficult to install and require extensive maintenance.

They have even replaced concrete posts, which are heavier and less practical to install.

Moreover, the market has focused on this product for its practicality during mechanical operations, especially the grape harvest. In fact, the machine passes easily among the iron posts and efficiently collects the grapes without having to reduce working speed.

Obviously, iron posts must be strong enough to withstand the stresses of the wire, but at the same time it must be adequately protected against rust, a natural phenomenon that leads to the progressive wear of the post until it breaks.

To respond to the many demands of farmers, the market offers products that have been galvanised using different methods and which therefore offer varying levels of performance in terms of effectiveness of protection.

There are two types of posts commonly found on the market: hot-dip galvanised posts and pre-galvanised posts.

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It is generally accepted that the former has a better quality coating than the latter, which inevitably is considered as being inferior, or even poor quality.

Is that right? Are we sure that hot-dip galvanised posts are better than pre-galvanised posts?

It is important to try to answer this question in order to clarify things as you make your decisions about which materials to purchase for a new vineyard installation.

So, getting back to the types of galvanisation, the “hot dip” variety bears the name used by operators to indicate posts that, after being produced, are galvanised by dipping them in a tub of molten zinc at a temperature of about 450°C so that it adheres to the metal.

This process gives the product a sufficient protection of the metal.

“Pre-galvanised”, on the other hand, is the term used to describe a post that is made using sheet metal supplied by the steelworks that has been galvanised ahead of time.

However, this type of processing has the problem of leaving parts that are free of zinc plating exposed to oxidation after shearing. But we’ll get back to that later.

A second problem is related to the fact that this type of processing has historically been used by producers with the primary objective of creating a cheaper product to compete against hot-dip galvanised posts.

Consequently, when price becomes the main focus, the choice of materials necessarily falls to low quality products, which in this case means galvanised steel with a very thin layer of zinc coating.

And here lies the first crucial point.

While it is not a mistake to state that generally speaking posts that are hot-dipped after processing are qualitatively superior to the pre-galvanised version, the pre-galvanised posts could be made with materials that make them equal to or even better than the hot-dipped version.

In the end, it is only a production and commercial choice and not a real limit of the pre-galvanising method, which to the contrary is much more versatile than hot dipping.

To understand better, you have to keep a few things in mind:

1-   The steel that is commonly called pre-galvanised is coated with a layer of zinc that is applied hot following the same exact procedure that is used to galvanise the posts individually after machining, it’s simply done directly by the plant and then the steel is supplied to the customer which, as in our case, uses it to produce the posts.

2-   Manufacturers of hot-dip galvanised steel posts are not able to adjust (increase or decrease) the amount of galvanising itself and consequently its thickness, which is therefore standard and common to all.

3-   In the case of steel that is pre-galvanised in the steelworks, on the other hand, the quantity of zinc applied to the sheet metal that will be used to produce the posts (and therefore the resulting thickness) depends on the request of the customer, in our case the producer of the posts. In fact, steelworks are able to galvanise sheets with varying thicknesses, even greater than those obtained by the producers of posts mentioned above using hot dipping.

4-   Furthermore, modern steelworks are also able to offer coatings made from metal alloys that are stronger and more durable than zinc alone, like zinc-aluminium and zinc-aluminium-magnesium alloys, which give the steel greater protection and longer life. These special coatings can only be applied in the steelworks.

5-   In addition to being more resistant to corrosion than zinc alone, zinc-aluminium-magnesium alloys also have a particular characteristic of self-healing that allows the coating to regenerate after machining, covering any parts that are uncoated after shearing.

This is what we know in theory, but in order to have scientific data that can give us definitive answers on the effectiveness of various products we have carried out special tests on materials coated with different zinc platings, combining the action of normal salt spray with that of chemical products that are now commonly used in vineyards.

Let’s take a look at what we found.



The results are obvious and clearly show the performance of each coating tested.

As you can see, the standard pre-galvanised post (Z275 coating) was very damaged by the tests.

The hot-dip galvanised post performed better, but contrary to common thinking about its effectiveness it is covered throughout by traces of rust.

The post with a zinc-aluminium-magnesium alloy coating, i.e. our EVO post, was found to be completely free of rust, responding in the best possible manner to the strict testing the materials were subjected to.

The tests therefore clearly show that the pre-galvanised post coated with the special alloy performed better than the hot-dip galvanised post.

To dispel any doubts, remember that these special alloys are only available when applied to the steel during pre-galvanising directly in the steel plant. It is not possible for a manufacturer of posts to use them for hot dipping their products.

So, returning to the title of this article, is it true that hot-dip galvanised posts are better than pre-galvanised posts?

The scientific evidence gives us two answers:

a)   Yes, if the pre-galvanised post is made of low quality steel and has a thinner coating than the hot-dip galvanised version.

b)   No, if the pre-galvanised post has a zinc-aluminium-magnesium alloy coating with a thickness equal to or greater than the hot-dip galvanised version. In this case, the pre-galvanised post performs much better.

Even behind this myth, therefore, there is scientific evidence that helps us to understand what the truth is.

We are always very careful when it comes to making statements, and we try every day to assess, experiment with and test new solutions to provide increasingly concrete answers to the market.

The basic objective is to inform and better understand the state of affairs before decisions are made, so that you can do so with greater awareness and conviction.

Of course we are available to provide you with all the information you need for the proper design of your installation and to help you in choosing the best materials.