The iconic Golden Gate Bridge is a large suspension bridge in the USA that connects the northern tip of the San Francisco Peninsula to Marin County. With a main span of 1,280 meters it crosses the1.6 kilometer strait that connects San Francisco Bay with the Pacific Ocean.

Such a structure must withstand extreme conditions. The bridge was completed in 1937, and its history is a good example of the impact of the choice of corrosion protection for the preservation of a steel structure.

The Golden Gate Bridge was originally protected against corrosion by a lead oxide primer and a lead-based topcoat. Beginning in the 1960s, the original paint was removed and replaced with a zinc silicate primer and vinyl topcoats, which provided better protection. Since 1990, acrylic topcoats have been used and a team of 38 painters now prevent corrosion by performing constant touch-ups.

This endless cycle of maintenance is certainly not cheap. The American Galvanizers Association estimates that if the structure had been fully hot-dipped galvanized from the start, the cost of construction would have been only 15% higher - while more than $1 billion in maintenance and repair costs would have been saved to date.

Since the 1930s, corrosion protection technologies have improved with one message that remains the same today: investing in effective corrosion protection from the start of a project pays off in the long run.

We are therefore pleased to offer you a series of articles on corrosion to better understand the subject and better protect yourself.

1-A chemical reaction

Corrosion is the physical-chemical interaction between a material and its environment that progressively leads to undesirable changes in the material's properties. This natural process transforms a chemical element into its stable form via a redox (reduction-oxidation) reaction. This reaction involves an exchange of electrons between two elements: a reducing agent that gives up a certain number of electrons and an oxidizing agent that recovers the electrons given up.

For iron, for example, the reaction occurs with water and oxygen simultaneously.

In a first step, iron oxidizes by releasing 2 electrons.

 Fe → Fe2+ + 2e

With water, the oxygen in the air is reduced by capturing the electrons released by the iron:

 O2 + 2H2 O + 4e-  → 4OH-

The balance of these two reactions (which are dependent on each other), is the following redox reaction:

 2Fe + O2  +  2H2 O → 2Fe2+  + 4OH-

The ions* Fe2 and OH- react together then to form iron oxides responsible for the appearance of red rust. As we can see on the above reaction, the iron is consumed to become re rust. We can then see the appearance of rust spots or even holes on the iron plate. This is called destructive corrosion.

Simplified diagram of the formation of red rust & example of rust on an iron plate

 *An ion is an atom (or group of atoms) that has released or captured one or more electrons. The conductive element where the oxidation reaction takes place (loss of electrons) is called anode and the conductive element where the reduction reaction takes place (gain of electrons) is called cathode.
With the example of iron, we understand that the material is consumed, and the mechanical strength of the corroded part is therefore degraded. Consequently, the system of which it is a part is also degraded. Therefore, this phenomenon must be considered to preserve a steel structure integrity.
Generally, this reaction being rather slow, the effects are only visible after several weeks or even months. The presence of salt or acid water can also act as a catalyst, since the conductivity of the water becomes greater, which facilitates the exchange of electrons. The reaction is thus accelerated. This is why in marine or "acidic" environments, corrosion appears more quickly. In general, there are different factors that influence corrosion.

There is therefore a classification of environments according to their corrosivity:

Corrosivity category and associated conditions according to ISO 9223

To effectively prevent corrosion, it is important then to choose a material according to the environmental conditions (indoor or outdoor) with which it will be in contact. Also, for each category of corrosivity, there are 3 durability classes that indicate the duration of resistance of a material: Low (2 to 5 years), Medium (5 to 15 years) and High (15 years and more). For example, Zinc Magnesium ZM310* offers corrosion protection from 5 to 15 years in a C4 environment (high corrosive atmosphere). In this example, the coating thickness is 24 µm.

Lifetime of Zinc and Zinc Magnesium (ZM) coatings

*ZM310 is the coating we have chosen to protect our new range of supports for outdoor applications: the MT modular system.

2-Why evaluate anti-corrosion needs is essential?

We perform extensive laboratory and field tests to evaluate the corrosion resistance of our products. Nevertheless, the choice of corrosion protection method for a specific application remains the responsibility of the user.

It is essential to choose the best protection at the design stage of a project, not only to facilitate inspections by inspection offices, but also to better avoid possible complications in the medium to long term. Poor corrosion protection can limit the life of components, involve costly corrective action or worse, catastrophic failures.    

Corrosion is a natural process that can have a significant economic impact. Approximately 20% of the world's steel production is lost each year due to corrosion. And even though it means a higher initial cost, opting for corrosion protection at the design stage saves money in the long run and conserves resources. It will also ensure the life of your steel part in its environment. Think of the painters who are still getting paid to work on the Golden Gate Bridge every day more than 80 years after the structure was completed!

3- What are the solutions?

The unalloyed steel from which most 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 considered in the design phase. Therefore, we offer a wide range of steel products adapted to resist corrosion using different processes:

• Phosphating
• Electro galvanizing
• Hot-dip galvanizing
• Continuous hot-dip galvanizing
• Sherardization

There are also other solutions such as multilayer coatings and stainless steels capable of meeting more specific needs. For more information on our protection solutions, please read our article on The basics of corrosion #3, Our anti-corrosion solutions

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

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

- Read our article on The basics of corrosion #4, Focus on Stainless Steel

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.