Localised oxidation

Localised oxidation

28 December 2020 0 By Facto Edizioni

/ by Aldo Avancini /

Aldo Avancini introduces a topic that it is not always easy to find answers for /

Ing. Aldo Avancini / Proposta Srl

The problem of localised oxidations on rides is a very complex one, often highlighted by the formation of rust or the removal of flakes of paint from the surface of metal structures immersed in water or in any case frequent exposed to wetting, even when protected. The complexity is due to the difference in the behaviour of the coating, and in some cases this can also be macroscopic.

Firstly, it should be noted that there are 2 questions we need to answer, namely:

whether the requested/proposed treatment applied to the metal structure was the most suitable for the intended application; 

having ascertained this, whether the treatment that certification was provided for was carried out correctly.

The first question can only be answered by looking at the quotation request/contract confirmation between the 2 parties; however, this documentation is often vague and not sufficiently complete, and thus a possible source of bitter surprises.

Taking as an example a hot-dip galvanisation cycle and subsequent painting, often considered to be a complete treatment, I would like to underline 2 aspects that take into account the specific problems; firstly, a certain period needs to elapse after mechanical cleaning of the galvanised surface, and secondly it is quite difficult to measure the thicknesses with certainty in the event where there are several layers. I can also add how accentuated oxidation is often seen on the edges of base flanges but not on the welds, and thus there is detachment of flakes of paint from the galvanised surface without any trace of adhesion.

With regard to the former, it must be emphasised that if there is no evidence of oxidation in the welds of components protected using the hot-dip process (the welds are normally the “mechanically altered” part in terms of metallographic composition and energy level, and therefore the first to be affected by oxidation) it can be confirmed that the treatment has been performed correctly.

It’s regarding the latter point that doubts arise from the visual evidence. Flanges or cut components may show hints of hardening on the edge or on the cut surface (local hardening may also occur on MT testing) especially if after the typical use of the cut, the higher energy value created on the edges is not lost.

In the absence of this, and in the presence of an electrolyte, even when diluted (pool water or slightly acid rain), the increased local energy value provides a means of transmission between two poles, which can be identified as mechanically altered metal>> electrolyte liquid >> non-mechanically altered metal, effectively creating a battery that, exploiting the higher energy of the edge with respect to the internal flange, generates the localised oxidation. One little known yet highly visible phenomenon can be seen in any construction site, with concrete rods, first bent for transport, then laid on the ground, that are rusted only in the bent part (deformed, therefore with a higher energy level) but not in the straight sections.

The second problem that arises concerns scarce or inadequate ageing after hot-dip galvanisation, a technology that actually releases gas (hydrogen) at the interface with the base metal. Where the coating is applied with significant thicknesses, and not adequately aged to allow the gas to escape, but immediately painted over, the gas still manages to flow through the galvanised layer but not through the paint, causing the paint to detach, while the galvanised surface is unchanged. This is known as “hydrogen embrittlement”, a common phenomenon, much more frequent than you think, especially when there are very short delays between the various stages. 

Taken from Games&Parks Industry December 2020, page 76

Ing. Aldo Avancini /  Proposta Srl / proposta_design@yahoo.it

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