In previous articles, we have seen how corrosion can impact the life of a structure and how it evolves with environmental conditions (Corrosion basics #1, What is corrosion?). We have also made an overview of the main types of corrosion (Corrosion Basics #2, The main types of Corrosion). In this third article, we will focus this time on corrosion protection solutions.

The unalloyed steel from which the majority of our fasteners and support systems are manufactured requires corrosion protection. In most environments, the corrosion rate of carbon steel (typically around 20 µm/year in a rural outdoor atmosphere and rising to over 100 µm/year in coastal environments) is generally too high for outdoor applications. This material loss is generally not taken into account in the design phase. We therefore offer a wide range of steel products adapted to fight against corrosion.

1-PHOSPHATING:

The steel is immersed in an acid solution containing metal phosphate salts (Zn, Fe). The solution reacts with the surface of the steel forming a microcrystalline layer of phosphates on the surface. This results in a rough surface with excellent oil retention properties. The oil applied for corrosion protection remains on the surface and provides protection during transport and slightly increases the overall corrosion protection. These products can only be used in dry indoor environments. This is the method of protection that is used for our single drywall screws.

2-COATINGS OF ZINC:

Zinc is an excellent choice for corrosion protection of steel. The corrosion rate of zinc is more than ten times lower than that of steel, with approximately 0.5 µm/year in rural/urban atmospheres and rising to 5 µm/year in coastal environments. In addition to reducing corrosion rates, zinc also provides cathodic (or sacrificial) protection of the underlying steel. When the zinc coating is degraded and the steel is exposed, galvanic corrosion takes place. Zinc, which is a less noble metal than steel, corrodes preferentially, keeping the exposed surface of the steel protected. The zinc layers are consumed evenly during "atmospheric" corrosion. Doubling the thickness of the coating also doubles the time until red rust appears on the steel substrate.
The suitability of zinc coatings is limited under conditions of permanent humidity or in the presence of high concentrations of industrial pollutants such as sulfur dioxide. Zinc is not resistant to the alkaline environment and is rapidly attacked by solutions with a PH >10. On the construction site, for example, the spillage of building material such as cement or fresh concrete should be avoided.

Lifetime of Zinc (Zn) and Zinc Magnesium (ZM) coatings in different environments

• Electro galvanizing :

Electro galvanizing is a simple and economically competitive process that consists of depositing zinc on a part by means of electrolysis: an electric current flow through an aqueous solution containing zinc ions and the steel part to be protected. This generates a zinc deposit on the steel substrate. Prior to this step, the parts are usually cleaned and pickled. This is an excellent way to protect small threaded parts due to the formation of dense, homogeneous coatings. The thickness of the coating usually ranges from 5 to 15 µm.
Due to the limitations of the achievable coating thickness, electro-galvanized parts without further corrosion protection should only be used in dry indoor conditions.

•  Hot-dip galvanizing :

The steel parts are immersed in a molten zinc bath. This technique is used to coat large parts with great length. Smaller parts such as bolts and pins are centrifuged after hot-dip galvanizing to remove excess zinc from the threads. Coating thicknesses range from 35 to 100 microns and depend on material thickness and steel composition. Immersion time is usually a few minutes. The molten zinc reacts with the substrate forming a layer of ZnFe alloy and a thinner layer of pure zinc on top (see figure below). Hot-dip galvanizing can be a solution for certain types of outdoor environments, but this depends directly on the thickness of the zinc coating and the exposure conditions.
 

Microscopic section of a hot-dip galvanized steel

• Continuous hot dip galvanizing (Sendzimir galvanizing):

Coils of sheet metal are continuously uncoiled in a molten zinc bath after the surface has been cleaned and specially annealed. Small amounts of aluminum in the zinc bath react with the steel surface to form an inhibiting layer a few nanometers thick, preventing the formation of a ZnFe phase. The coating consists mainly of pure zinc and varies in thickness from 10 to 70 microns on both sides of the sheet. Zinc alloy coatings such as Zinc Magnesium (ZM) can be produced by continuous hot dip galvanizing. In general, this type of coating consists of 2 to 4% aluminum and magnesium and its ability to protect against corrosion is about twice that of a pure zinc coating for the same amount of coating.

• Sherardization / thermal diffusion :

Sherardization is a method of zinc coating that uses a thermal diffusion process. Steel parts are placed in a drum containing Zn powder and heated to temperatures above 320°C. The zinc is not molten, and the coating is formed by thermal diffusion of the Zn powder into the steel parts. The coating thickness can be up to 45 μm. These coatings are mainly made of a ZnFe alloy, which provides very good corrosion protection that can be compared to hot-dip galvanizing for the same thickness. Even on complex threaded parts, this process produces tough, uniform coatings.

3-MULTILAYER COATING (DUPLEX COATING):

When the corrosion protection provided by the metallic coating is not sufficient, the parts can be protected by additional coatings, mainly organic paint with or without metallic flakes.

One example is the multi-layer coating on fasteners, consisting of an electroplated* Zn alloy coating and an additional organic topcoat (see figure below).

*Electrolysis is a process that generates chemical reactions using an electric current.

Illustration of multi-layer coating on fasteners and connectors

4-STAINLESS STEEL:

Stainless steel is a steel to which chromium atoms have been added (representing at least 10% of the composition) whose remarkable property is to be little sensitive to corrosion.

To learn more about stainless steel, feel free to browse our article Corrosion Basics #4, Focus on Stainless Steel.

5-SELECTION OF TREATMENTS:

As an indication, it is possible to establish a classification of all the anti-corrosion solutions using guidance from standards such as EC2

Environmental conditions according to EC2

 To learn more about our corrosion protection options, please visit our AskHilti platform:

- Read our article on the basics of corrosion #1, What is corrosion?

- Read our article on the basics of corrosion #2, The main types of corrosion

 You are also welcome to ask us for support: simply leave a comment or post your question in the community, or improve your knowledge and skills via our Webinars or training sessions.