Photovoltaic bases made of pre-galvanized sheet metal. Is it a good idea?
In Greece, unlike all the developed PV markets of the world, the practice of installing photovoltaic bases is quite widespread, where while the piles are hot-dip galvanized after construction (according to EN ISO 1461) the superstructure is made of pre-galvanized sheet metal. ELVAN S.A.V.E. recommends the construction of fixed photovoltaic bases (piles and superstructure) exclusively and only hot-dip galvanized after construction according to EN ISO 1461. The main feature of hot-dip galvanizing after construction is that, unlike pre-galvanized materials, it provides a very long service life without any maintenance.
The process of hot-dip galvanizing after fabrication is as follows: after the end of any treatment, the steel is immersed in a bath containing molten zinc at a temperature of 450 °C. This results in hot dip galvanizing being an alloying process rather than simply coating the steel with zinc. In addition, all holes and edges are protected (since the galvanizing is done after construction). Thus, hot-dip galvanized steel acquires particularly high resistance over time against oxidation even in the most adverse environments.
In contrast, in pre-galvanized sheet steel, the steel passes through a zinc bath in a continuous flow and is rolled into a roll. In this way it obtains a thinner layer of zinc (about 1/4 compared to hot-dip galvanizing). However, since the steel is treated after galvanizing at the cut points and holes, there is no anti-corrosion protection.
Zinc coatings in various forms have been used to protect steel against corrosion for over 150 years. He has thus collected a very large amount of data on the strength of hot-dip galvanized steel over time in various environments.
For each different environment, the lifetime of a galvanized structure is proportional:
- of the initial thickness of the galvanization and
- of the annual reduction in galvanizing thickness due to atmospheric and soil conditions.
To estimate the annual reduction in galvanization thickness we must first consider the external environment in the area where the photovoltaic park will be installed. According to EN 50929 (Corrosion likelihood of metallic materials when subject to corrosion from the outside – Classification of environmental conditions) the areas are classified into 6 categories: C1, C2, C3, C4, C5-I and C5-M. Photovoltaic parks in Greece are usually installed in C3 areas (rural – medium load – with an annual reduction in galvanizing thickness of 2.1 µm).
But why are fixed photovoltaic bases made of pre-galvanized sheet metal not suitable in any case?
For two reasons:
- the galvanizing thickness on a pre-galvanized sheet is at best 16.5 µm (quality Z 275). This in a C3 area gives us a life time of 7 years, which clearly falls short of the 20 year life time requirement of a PV park.
- the points where the pre-galvanized sheet has been cut are left without any anti-oxidation protection. In these areas, oxidation develops almost immediately, which penetrates under the galvanized surface. The result is an even shorter lifespan. For this reason and DIN 55928 – 8 in paragraph 7 emphasizes that in sheet thicknesses of more than 1.5 mm it is necessary to have protection also at the points where the sheet is cut and the only way to achieve this this is hot dip galvanized after fabrication.
Practically speaking, a pre-galvanized base is certain to show oxidation within a relatively short time after installation, even within six months, especially in the holes and cut points, where there is no zinc for anti-oxidation protection. The question is whether this oxidation will cause the structure to fail within 20 years or not. And the even more important question, addressed to investors, E.P.C’s, insurers, etc. is: “Are you willing to risk it, for a savings of 1.7% on the total project budget?”