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CROSS Safety Report

Possible galvanising concern

Report ID: 48 Published: 1 November 2006 Region: CROSS-UK

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Overview

A reporter has a question about using galvanised steel poles for the members of dome structures.

Key Learning Outcomes

For civil and structural design engineers:

  • This report emphasises the need to utilise the expertise within the supply chain at the design stage, particularly from fabricators and galvanisers, in order to identify possible problems and find suitable solutions

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A third reporter has a question about using galvanised steel poles for the members of dome structures. The intention is that steel poles will be pressed to form the end connection serrations in the form of a ‘fir tree’. In this cold forming process, there will be some residual stress at the serrations and the flattened part of the pole. 

Before the hot dip galvanising, the pole material needs to be cleaned by pickling, which is an acid bath. Thus, the two components for stress corrosion cracking are present - stress from cold forming and then the acid giving the corrosive environment. Even though the pickled pole members will be cleaned or flushed and purged with hot air, the acid can remain in the small crevices that are present on the surface of the pole. 

When dipping into the hot zinc bath, all the crevices will be covered by the molten zinc and after the dipping, it is not possible to detect the small crevices by NDT. This is the inherent problem of any galvanised steel component if there is any cold forming beforehand. If the serrations are scratched in the installation process, the exposed steel part under high stress in the fir tree root may again be susceptible to stress corrosion.

The reporter wonders if this concern is justified and if so whether it would be related to stress corrosion or liquid metal assisted cracking.

Expert Panel Comments

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In the light of the two previous reports (45 and 49) the concern may well be justified. This case emphasises the need to utilize the expertise within the supply chain at the design stage, particularly from fabricators and galvanisers, in order to identify possible problems and find suitable solutions.

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