Steel Protection By Hot Dip Galvanizing & Duplex Systems

Information courtesy of: HOT DIP GALVANIZERS ASSOCIATION SOUTHERN AFRICA

Choice of Rust Prevention Method

When choosing a rust prevention method for a steel component or structure, there are many technical issues to be addressed. The environment in which the steel component or structure is to work must be analysed carefully. The need for handling, transport, fabrication and final erection require careful consideration.

The reliability factor of a coating may be defined as the extent to which its physical, chemical and mechanical characteristics can be consistently realised during and after application.

The reliability factor determines the overall cost-effectiveness of a coating in an environment.

There are numerous paint systems for steel and a wide range of possible specification and application variables. These variables can substantially reduce the performance of a given system and therefore its economics. By contrast, the hot dip galvanizing process is simple, standardised and virtually self-controlling, governed mainly by the laws of metallurgy. As a result it is inherently reliable and predictable.

Table 1 summarises factors determining the reliability of typical paint systems for steel, and for hot dip galvanizing. The reliability factor for hot dip galvanizing is shown to be superior, mainly because it is not influenced by most of the variables which can reduce the ultimate performance of typical paint systems.

Figure 5. Hot dip galvanized handrail and vastrap stairs.

Since paints are available in countless variations, with different properties, conditions and demands being variable in practice, a comparison with actual parameters is often advisable.

Further, an economic study of different prevention methods should be undertaken. It is important that the choice of method be based not only on initial costs but also on packing costs for transportation, touch-up painting after erection and future maintenance costs.

A good guide to the selection of corrosion prevention methods in different environments can be found in SABS ISO 14713 - Protection Against Corrosion of Iron and Steel Structures -Zinc and Aluminium Coatings - Guidelines and in SABS ISO 12944 Parts 1 - 8 - Corrosion Protection of Steel Structures by Protective Paint Systems.

The environmental classifications defined in these standards concern only the environment in which the structure will function. However, transportation, storage and erection environments can change the environmental classification, and therefore the choice of rust prevention method.

Figure 6 serves as a guide for comparing the technical characteristics of different types of coating.

It should be noted that, even if zinc and paint are applied with the same objective - to prevent corrosion - they act in completely different ways. The zinc coating corrodes from the surface in towards the steel, and gives cathodic protection in the event of damage to the coating. Corrosion does not occur between the zinc coating and the steel.

Conversely, paint coatings are often destroyed through the development of a layer of rust between the paint and the steel. Since the paint coating gives no cathodic protection, rust is able to penetrate further beneath the paint coating once the coating has been damaged.

Figure 6. Comparison between the properties of different surface coatings.

Table 1 - Comparison of the properties between a paint system and hot dip galvanizing.

Table 2. Zinc coatings compared in tennis of coaling thickness and relative life expectancy

COMPATIBILITY OF GALVANIZED COATINGS WITH VARIOUS MEDIA

Compatibility of lion dip galvanized coatings with various media is summarised in the table below. Further specific information is available front Hot Dip Galvanizers Association Southern Africa.

Table 3. Compatibility of hot dip galvanized coatings with various media.

Table 4. Properties of zinc.