Technical Info Sheet #30
Nothing offers more security than hot-dip galvanized steel. There are no hidden defects, and hot-dip galvanizing has been the champion of corrosion resistance for more than 150 years. Still, even a hot-dip galvanized coating does reach its end sometime. We know that this moment usually never comes until after many decades. Just a one-time investment and no more maintenance is required. No wonder many choose hot-dip galvanized steel. So why is it that the zinc layer gradually disappears? We will explain in this technical info sheet.
People often talk about “galvanizing.” However, there are several zinc application methods, each with its pros and cons. It is therefore important to understand what these differences may lead to. See also ‘[Ver-zin-ken] Different techniques for galvanizing’ and Technical Data Sheet 11: Zinc Application Methods.
Hot-dip galvanizing is a metallurgical process that occurs when we immerse steel in molten zinc. The coating is created by Fe-Zn diffusion at the steel surface, followed by the formation of Zn-Fe alloys that are “metallurgically anchored” to the steel surface. Excellent adhesion and wear resistance is the result.
Once the galvanized steel is assembled and exposed to the atmosphere, an additional protection of the galvanized coating occurs: the zinc patina layer. It is this patina layer that inhibits the reactivity of the zinc surface and thus prevents rapid corrosion of the zinc. If this patina layer is somewhat porous due to aggressive substances in the air then there will be an accelerated decrease in the zinc layer, whereas with a closed patina layer there is hardly any decrease. In short: atmospheric conditions determine the period during which the zinc provides protection for the steel.
After decades, there comes a time when the object is renovated. This may be because major maintenance is being performed and one would also like to see the galvanized materials “like new.”
It is not necessary to make new handrails, for example. If one considers the current handrail still suitable, then renovating it is sufficient. It is a huge advantage especially from an environmental and carbon footprint point of view. After dismantling and cleaning the object, you deliver it to your galvanization plant to apply a new zinc layer.
When re-galvanizing an object, it is important that it is delivered “clean”. Things such as; stickers, paint, dirt, cement, concrete and grease must definitely be removed, as well as any plastic caps or plugs as otherwise the object cannot be treated. If the structure consists of hollow profiles, it will be necessary to look closely at whether the inside of them is also sufficiently clean. One will also have to review the size of the countersink holes. Over the years, the requirements stated in standard EN-ISO 14713-2 have changed somewhat.
PUBLIC WORKS AND RENOVATE THE GUIDE RAIL
Rijkswaterstaat (RWS) would like to work climate-neutral, circular and emission-free. Reusing guide rails, popularly called guardrails, contributes to that ambition. For that reason, there has been a lot of communication with stakeholders since 2019, and from that has emerged the desire to opt for reuse (sometimes called “Reno”) instead of scrapping guide rails. It may be that the condition of the galvanized guide rail is still such that immediate redeployment is possible. Another option is to refurbish the guide rails and apply a new corrosion-resistant zinc coating. To get the market to cooperate in realizing this
“clean” method, the tender policy has been modified to make reuse interesting despite a higher tender price. RWS rewards the contractors with the lowest environmental impact with a notional discount via the so-called Environmental Cost Indicator (EQI). This makes it attractive
for contractors to work with reusable guide rails already during the tender phase of a work.
In most cases, the galvanized coating is first removed after which a new one is applied. In the meantime, techniques have been developed whereby this removal of the existing zinc layer is no longer necessary. For the time being, this is only possible with serial profiles such as a guide rail to which no other parts are welded.
Renovation of guide rails technically and economically feasible, with environmental gains
The renovation of guide rails can be done responsibly in practice. Renovating and reusing gives significant environmental benefits, with savings on environmental costs up to 70%. In addition, renovation is cheaper than producing new and fits it within the strict safety requirements that apply to guide rails. This is according to a study by Rijkswaterstaat, organizational consulting firms TwynstraGudde and LBPSight, contractors involved, suppliers of guide rails and galvanizers. Making guide rails circular is one of the ways for Rijkswaterstaat to work circularly by 2030 and thus minimize primary raw material use. The exploration of circular guide rails examined the entire renovation chain
taken, giving used guide rails a next life by disassembling the rail, cleaning it cleaning, de-galvanizing and re-galvanizing to then be placed back along the road.
When re-galvanizing, remember that a new layer of zinc is applied to the steel object. So; the materials behave like new again and will again protect the steel from corrosion for decades. Did you know that this can be done more often? At least 10 times! If we consider this method as “major maintenance” then an object such as a railing, for example, can last 500 years under normal conditions! There is increasing attention in Europe for the reuse of steel construction parts. In the Netherlands a technical instruction has been issued by NEN: NTA 8713. There is also a European technical specification in force: CEN/TS 1090-201.
The environmental impact of re-galvanizing is extremely low compared to the re-manufacturing and preservation of the parts. Indeed, steel production and associated transportation and operations are unnecessary. The product is already there! Unlike paint layers released in the form of waste during blasting and sanding, the zinc is dissolved in acid and can be recovered quite easily. Zinc is zinc; it is a metal that always retains the same properties no matter how many times you use it and reclaim it.
EN ISO 1461
Coatings applied by hot-dip galvanizing to iron and steel objects – Specifications and test methods.
EN ISO 14713 part 2
Zinc coatings – Guidelines and recommendations for the protection of iron and steel in structures against corrosion – Part 2: Hot dip galvanizing
NTA 8713
Reuse of structural steels
CEN/TS 1090-201
Performance of steel structures and aluminum structures – Reuse of structural steel.
TECHNICAL DATA SHEET 10
Corrosion resistance of hot-dip galvanized steel
TECHNICAL DATA SHEET 25
Zinc patina: origins and protection
