Corrosion part 1 - Basics
This introductory article explains the basics of corrosion and why it is important in construction. Corrosion is defined as a chemical process where metals react with their environment and gradually break down, which can reduce strength and shorten the lifespan of structures.
Introduction
One of the most common topics we discuss with designers and contractors in the design phase and in production is corrosion. Corrosion is an area that should be taken into account when choosing steel products, for all environments that are not dry indoor environments. Choosing the wrong product, treatment or alloy can have major consequences.
The most obvious is that the building, or the specific building part's lifespan or maintenance interval, is shortened. Choosing a product with higher corrosion resistance than necessary can also result in unnecessary costs for the project. We will therefore navigate through a series of articles through different types of materials, corrosion types and choosing the right product.
What is corrosion?
In this first introductory part, we will go through the basics of corrosion. In the following articles, we will delve into the following areas - "Choice of galvanised product", "Choice of stainless steel product", "Zinc Magnesium" and "Galvanic corrosion"
The first thing many people think of when they hear corrosion is rust. Rust is a phenomenon that occurs when corrosion occurs on certain types of materials. But there are also other types of more difficult-to-detect corrosion such as spot corrosion that can break down the material from within. Galvanic corrosion is also to be taken into account, which can have a significant effect on the corrosion process if different metals are mixed.
What is meant by corrosivity class?
Corrosion is a chemical process in which metals are broken down through a reaction with the surrounding environment. This can lead to damage to the material, lose its strength and, in the long run, to a structure failing. Corrosion occurs when we have a combination of metal, moisture and oxygen - but the corrosion process can also be accelerated by, for example, sulphur, chlorides and dirt (which retain moisture to the material).
What types of materials have the highest corrosion resistance?
There are a number of different material compositions (alloys) and surface treatments used in steel construction. Here we list some of the most common varieties:
Carbon steel Untreated steel where the composition normally consists of iron and a proportion of carbon. Common products are reinforcement and untreated steel profiles, such as beams. These are often rusty already on delivery and are sometimes also called black steel. Untreated carbon steel is rare to use unprotected in structures. Usually, these structures are painted or treated afterwards if they are to be visible.
Electrogalvanizing Electrogalvanizing is a process in which a thin layer of zinc is applied to the surface of the steel. The product is placed in a zinc solution and an electric current is applied, which causes zinc ions to stick to the surface of the steel. This thin layer provides a corrosion resistance that does not affect the other properties of the product. Electrogalvanized products, such as fasteners and mounting components, are mainly used in dry environments with corrosivity class C1 or C2 depending on service life requirements. Electrostatic galvanizing is a cost-effective corrosion protection treatment and is therefore common for many types of products that are installed in indoor environments, such as screws, fasteners and installation suspension.
Hot dip galvanizing Often referred to as galvanized steel. Hot-dip galvanizing is also a surface treatment in which a surface layer is applied to the steel. In this process, the steel is dipped in a zinc bath. This can be done in several rounds to make the zinc coating thicker and thus provide greater corrosion resistance. Hot-dip galvanizing is common for products and structures that are to be installed in outdoor environments in corrosion class C2-C3, such as railings, posts, mounting rails and fasteners. Hot-dip galvanizing is also a relatively cost-effective corrosion resistance treatment.
A disadvantage of hot-dip galvanizing is that products and structures with a more complex design can limit the thickness of the coating layer. It can be difficult to ensure that the layer is evenly distributed and a thick layer of zinc can also affect the properties of the product. A concrete example of this is that the maximum coating of a hot-dip galvanized expander bolt is 40-45 mu for the nut and thread as well as the expansion segment to work. In the case of other types of hot-dip galvanized structures such as beams, railings, and posts, these are often treated with a minimum of 115 mu. In cases where a hot-dip galvanised beam or railing is combined with a hot-dip galvanised fastening, it should be noted that the expected corrosion resistance of the beam is significantly greater than that of the fastener. It may therefore be inappropriate to choose a hot-dip galvanized fastener in such a case - unless there are additional protective measures and/or inspection and ongoing maintenance.
Zinc Magnesium - and other coatings Treating products and other structural components with zinc magnesium has become increasingly common in recent years. It is a surface treatment in which a proportion of aluminium and magnesium are added to zinc treatment. With this method, it is possible to provide better corrosion resistance with a thinner layer than with hot-dip galvanizing. Nowadays, zinc magnesium is a commonly used material for metal roofs and other sheet metal details that make up parts of the shell of a building. For a number of years, Hilti has been manufacturing zinc-magnesium outdoor suspension systems to meet the tough demands of outdoor environments, infrastructure projects and industries. Zinc magnesium is often a suitable choice in corrosivity classes C3, C4 and C5.
Processing products in multiple layers with different coatings is increasingly common. Due to the large difference in cost between galvanized and stainless steel products, this method has quickly become increasingly common. Duplex-treated products, where different coatings are applied in layers to create greater resistance than for a zinc-plated product, are one method. An example of a product is Concrete Screw HUS4-HF, which can be a relevant alternative if the choice is between a hot-dip galvanized or stainless steel product.
Stainless steel is a collective name for certain steel compositions. Alloys with at least 10.5% chromium are normally referred to in Sweden as "stainless steel". In other countries, these alloys are often referred to as "spotless steel" as the surface does not normally rust. However, even these steel grades can corrode, often through spot corrosion that is significantly more difficult to predict and distinguish.
Point corrosion is a phenomenon in which the surface layer of steel is broken and small holes are created. The attack can then penetrate the steel and continue the corrosion process without it being visible on the surface. It is therefore important to make conscious choices when it comes to stainless steel as well. Stainless steel in various compositions is widely used in corrosivity class C4 and above. But even in corrosivity class C3, it can sometimes be reasonable to consider a stainless steel product.
How to choose the right coating or alloy?
Choosing the right alloy or coating requires both knowledge of the specific environment and a defined desired service life. Routines for inspection, maintenance and cleaning can also be crucial factors.
There are different reasoning as to whether the products themselves should be classified or stamped as, for example, "Approved in a C4 environment". It can be considered a misleading evaluation method as this type of tests and standards do not give the whole picture. In many cases, for example, it may be time-limited guarantees that do not correspond to the intended lifespan of the building part. The performance of these tests has also been criticized for being misleading, as the laboratory environment in which they are performed is not the same as the intended environment.
Another more tried and accepted method is to instead follow SS-EN ISO 12944 and SS-EN 1993-1-4 standards and norms. In "Corrosion Part 2 - Selection of Galvanized Product" and "Corrosion Part 3 - Selection of Stainless Steel Product" there is an opportunity to further delve into how these standards can be applied. The image below provides an overview that can simplify when choosing a coating or alloy.
