Introduction
A poultry farm experienced premature failure (2 years) of IBR pure zinc galvanised roof sheeting. This was found to be due to severe attack by high-sulphur condensate on the inside surface.
Investigation
A product sample was chemically analysed. A material selection was conducted to find roof sheeting which would offer more resistance to the condensate.
Results
Product analysis
Analysis of the product shows high sulphate levels which can promote the growth of sulphate-reducing bacteria, leading to corrosion. Studies have found that the contribution of sulphate-reducing bacteria to corrosion of carbon steel, zinc and Zn-55 Al coatings is insignificant.
Material selection
There are two types of roof sheeting that are suitable for this application:
Mild steel with Coating
A number of continuous hot dip coatings are available for roof sheeting (Table 1).
Table 4: Comparison of Hot Dip Coatings
| Coating Type | Description | Roof sheeting Application |
| Zinc Coating(Galvanised) | Most commonly used coatings | Used for roof sheeting applications, entry level product |
| Zinc Iron Coating(Galvanneal) | Usually used in the painted condition; has excellent abrasion resistance | This coating and is used for autobody panel, stairs and walkways rather than roof sheeting |
| Aluminium Type 1 Coating | Type 1 coatings are usually up to 30 µm thick and are an aluminium silicon alloy | Used for automotive applications for exhausts rather than roof sheeting |
| Aluminium Type 2 Coating | Type 2 coatings are usually 30-50µm thick; excellent corrosion resistance but can be poor in salt water; rusting of cut edges in industrial and rural environments | Used for roof sheeting applications |
| Zinc – 55 Al Alloy Coating(Galvalume, Zincalume) | Zinc coating with additions of aluminium; has excellent high temperature resistance | Used for industrial roof sheeting applications. |
| Zinc – 5Al Alloy Coating(Galfan) | Zinc coating with additions of aluminium; more corrosion resistant than zinc coatings in severe marine environments only | Used for roof sheeting applications |
| Pb-Sn Coating(Terne) | Lead-tin alloy | Used in automotive industry |
Aluminium Type 2 and the Zn-55 Al Alloy coatings are the most appropriate for roof sheeting with high corrosion resistance in industrial environments.
Zinc coatings protect the steel by initially forming a barrier. It provides galvanic protection by acting as a sacrificial anode on cut edges and voids. The zinc corrosion products then form a secondary barrier. Sulphates are found to be highly corrosive to zinc.
Aluminium coatings exhibit good barrier properties due to its relatively low corrosion rate in most atmospheres.
The Zn-55Al alloy coating combines the barrier protection resistance of aluminum with the galvanic corrosion protection of zinc.
For the same coating thickness, the corrosion resistance of Zn-55Al is 2-4 times greater than that of pure zinc.
The corrosion resistance of aluminium is greater than the Zn-55Al coating. It is believed that it is more expensive than the Zn-55Al, and is not available commercially in South Africa.
Solid Metal Roof Sheeting
Solid metal roof sheeting is available in stainless steel and aluminium. Aluminium alloys are far less reactive than the stainless steels.
Stainless steel is susceptible to pitting corrosion and the presence of sulphates may pose a risk for some stainless steels. Sulphate reducting bacteria (SRB) convert the sulphates to highly corrosive sulphides. These react with the metal surface allowing a corresponding localised corrosion in the form of pitting. The only grade of stainless steel available for roof sheeting appears to be AISI 430, which has poor corrosion resistance compared to many other grades.
The aluminium alloy A3004, unpainted at 0.9mm gauge thickness, has been recommended for this application by a technical consultant to the Aluminium Federation of South Africa. It has been commonly used for roof sheeting in industrial and coastal applications. A corrosion expert has stated that aluminium has little or no reaction to sulphates.
The A3004 Aluminium roof sheeting is the most suitable roofing available, offering more corrosion protection than AISI 430 stainless steel roof sheeting and the hot dipped coating method.
