What is corrosion, and how do I better protect against it?

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 large structure needs to withstand extreme conditions. However, the bridge was completed in 1937, it's story is a great example of how choosing the right corrosion protection is a critical factor when preserving structural steel work. The Golden Gate’s original corrosion protection was provided by a coat of red lead primer and a lead-based topcoat, which was topped up as needed. From the 1960s, protection began to be improved as the original paint was removed and replaced with a zinc silicate primer and vinyl topcoats. From 1990, acrylic topcoats have been used and a team of 38 painters now prevent corrosion by continuously touching up the paintwork.

This never-ending cycle of repainting doesn’t come cheap. In contrast, the American Galvanizers Association estimates that, if the structure had initially been completely hot-dip galvanized, the cost of construction would only have been 15% higher – with the added benefit that some $1 billion of maintenance and repair costs would have been saved to date.

How do I choose the right corrosion protection?

Corrosion protection technologies have improved since the 1930s, but the message today is still the same: investing in the right corrosion protection at the start of any project will likely pay off in the long run.

That said, evaluating and choosing the appropriate materials is not always an easy task. You need to consider a number of different factors when it comes to corrosion assessment. These typically include the type of corrosion and related classes, environmental parameters and resistance categories.
To assist you, we’ve created this series of articles to help raise your general awareness of what corrosion is all about and help you to become more informed when choosing the right protection.

Basics of corrosion

Corrosion is the physicochemical interaction between a material (typically metal) and its environment, which results in undesirable changes to the material’s properties. These may lead to significant functional impairment of the material, the environment, or the technical system of which a material component forms a part (see ISO 8044:2010). The figure below shows the chemical reactions of iron during corrosion.

Understanding different types of corrosion

Uniform corrosion is the most conventional form of corrosion. It leads to an even distribution of the corrosion products (e.g. red rust on steel). The extent of this corrosion reaction can usually be well estimated. The rate of corrosion is given in micrometers per year (μm/a). Using these average values, it is possible to calculate the life expectancy of a component and thus enhance its working lifetime by increasing its thickness.

Pitting corrosion is a localised corrosion that leads to the creation of small holes or ‘pits’ in the metal. The corrosion process starts with a localised break-down of the passive layer. The holes can corrode very rapidly in depth, while the rest of the surface remains undamaged. Stainless steels are particularly susceptible to pitting corrosion, as are other passive metals such as aluminum and their alloys.



Hydrogen-induced cracking involves a dual process of corrosion and straining of a metal due to applied or residual stress. This type of corrosion is a combined mechanical and electrochemical process that results in cracking of certain materials. Internal stresses in a material can be enough to result in failure.

Stress corrosion cracking can be induced by the formation of chlorides or hydrogen during the corrosion reaction (H ind SCC). A dangerous source of hydrogen comes from the corrosion of zinc coatings. This limits the use of zinc-coated, high-strength products to dry, indoor environments.

Crevice corrosion is a localized form of corrosion occurring in cracks or crevices formed between two surfaces made of the same metal, different metals or even a metal and a non-metal. It is initiated by the restricted flow of oxygen from the air into the crevice area, leading to different concentrations of dissolved oxygen.

Bimetallic (galvanic, contact) corrosion occurs where two dissimilar metals coupled in a corrosive electrolyte. Generally, the less noble metal acts as the anode and dissolves, whereas the more noble part is the cathode and corrodes slower than alone. A positive example is the way zinc protects carbon steels and low-alloyed steels. Zinc is the less noble metal, which protects steel sacrificially by becoming corroded instead.

Why is corrosion assessment critical?
Generally, in typical construction to require regular inspections or maintenance due to corrosion is often met with resistance. Therefore, it is essential to choose the best corrosion protection for the project during the design stage to not only assist with inspections and maintenance but also to better avoid future issues. As you can see from the examples below, the wrong corrosion protection can lead to a limited lifetime of components, expensive remedial action or even catastrophic failures.

An electroplated HDA anchor after 7 years in a coastal climate. Its typical lifetime of 50 years will not be reached.

Corrosion protection to help ensure lifetime and cost savings

Corrosion is a ubiquitous natural process that can have a huge economic impact. About a fifth of the world’s annual steel production simply goes towards replacing parts damaged by corrosion. Even though it may involve higher up-front cost, the simple step of considering corrosion protection during the design stage can help save a significant amount money and resources in the long run.
Protecting materials against corrosion is essential if you want to better ensure the material and its overall application meets or exceeds the desired lifetime. Environmental conditions, oxidation, abrasion and several other factors will compromise the lifetime of an installation, but with the correct protection plan you can help guard against these – and subsequently avoid incurring additional repair or replacement costs.
Think of the painters still paid to work daily on the Golden Gate Bridge more than 80 years after the structure was finished!

In the next article, we will guide you through the behaviour of materials used to protect the most common applications from corrosion.

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