Corrosion part 2 - Selection of galvanized product
This article focuses specifically on surface corrosion, or rust as it is often called, the most common form of corrosion to consider when choosing a galvanized product. In future articles, we will also go through spot corrosion ("Choice of stainless steel product") and galvanic corrosion.
Introduction
Corrosion is a crucial factor to consider when building in steel. In order to meet the entire life cycle of the structure, all components must be chosen carefully, often taking into account both cost and environmental impact. In this article, we will explore current standards and norms that can hopefully provide guidance as well as valuable knowledge to make the best choice possible.
Hilti has extensive experience of product selection in different types of environments and also conducts its own research and development in corrosion resistance. Thanks to this, we have been able to support with the choice of product in thousands of projects around the world.
Choice of zinc plated product
A common question we get from designers and contractors is:
"Is this product approved for a C4 environment?"
"It is not prohibited to use in a C4 environment" - is then often the answer we give.
There are products that are labelled as, for example, "C3 approved" or "C4 approved". This type of classification can be considered misleading because the tests that form the basis for the "label" are often based on an evaluation with time-limited guarantees. It can be an evaluation of corrosion resistance for, for example, 10 or 15 years.
It is important to note that for most uses in buildings, and other larger structures, 10 or 15 years is rarely enough. Hilti has therefore chosen not to evaluate its products according to this method.
Standards and Norms - Guidance in the selection of coating or alloy
We consider choosing a product based on the current environment and the lifespan required for the building part to be a more appropriate approach. Maintenance and inspection should also be factors to consider.
With transparency regarding the surface coating thickness of products (often zinc) or stainless steel products' alloys, it becomes easier to optimize product selection based on the specific requirements and conditions that prevail.
SS-EN ISO 12944 and SS-EN 1993-1-4 help to support these choices as they describe the corrosion resistance of galvanized and stainless steel grades.
Thickness reduction of pavement - a decisive factor
SS-EN ISO 12944 describes how different environments affect a product's zinc coating. It is listed as "thickness reduction zinc", in micro-meters (μm). As a concrete example, there is a very low reduction per year in a C1 environment, while in a C4 environment it can amount to 4.2 μm per year. To put this into perspective, a hot-dip galvanised threaded rod, or expander, usually has a coating of about 40-45 μm. This means that it can withstand corrosion attack for up to 450 years in a C1 environment or at least 10 years in a C4 environment. A railing in a C4 environment often has at least 115 μm of zinc coating.
Should a hot-dip galvanised fastening with a significantly thinner coating be chosen, it means that the service life is shortened - or the maintenance interval for the structure must be increased. A better alternative may then be to instead choose a stainless steel attachment and achieve the desired lifespan - without frequent intervals of maintenance and cleaning. Why an expander or threaded rod does not have a thicker layer of zinc is because it would negatively affect the usability of the product - thread and expansion segments do not work optimally with a coating that is too thick.
Other aspects - galvanic corrosion
Combining metals of different nobleness can affect the corrosion process and create galvanic corrosion. An example of this could be the case of the hot-dip galvanized railing that is fastened with a stainless steel fastener. For this case, it is considered to be the best solution because the mass of the railing is significantly greater than that of the fastener and it will therefore not be significantly affected by the galvanic currents. Another solution could be to separate the materials using a plastic tray or other method, to completely avoid the galvanic phenomenon. In our article "Galvanic corrosion"we will describe this in more detail.
The Hilti Corrosion Handbook provides further insight into the subject of corrosion. You can also contact our engineers at the Technical Centre who are ready to support you with whatever you need help with.