SANS 121:2000, together with ISO 1461, has been superseded by SANS 121:2011. SANS 121:2000 or rather, the current standard of SANS 121:2011, is the national standard for hot-dip galvanised coatings on fabricated or manufactured iron and steel articles. It covers the specifications and tests for these standards. Accredited hot-dip galvanising plants adhere to the requirements of SANS 121:2011 and its predecessor, SANS 121:2000, as well as ISO 1461.
The Difference Between SANS 121:2011 and ISO 1461
SANS 121 is the South African national standard while ISO 1461 is the International Standards Organisation (ISO) standard for hot-dipping galvanised coatings on steel and fabricated items. The ISO develops the specifications and testing methods against which the products of the galvanising plants are measured. The methods applied by accredited plants thus meet the strict requirements of ISO 1461 as well. To ensure the quality and structural integrity of products, galvanising plants must be able to hot-dip galvanise products according to the standard that a client requires, whether it is ISO 1461, SANS 121, or ASTM A123 The latter is the North American standard to which galvanisers adhere in their hot-dipping methods for steel.
That said, meeting the requirements of ISO 1461 implies meeting the requirements of SANS 121, though there are a few differences. The standards mentioned are in place to provide the benchmark for testing the quality of the galvanisation and the resulting products. The thickness of the coating is, for example, tested using an electronic or magnetic thickness gauge as the main method for establishing whether the coating is of the required thickness. As an alternative, the gravimetric test can be used when the magnetic-gauge method cannot be used effectively.
What Happens Before Galvanisation
Before galvanisation can take place, the steel must undergo a thorough chemical cleaning process for the removal of debris, scale, oily substances, and rust from its surface. This is essential to make sure that the zinc coating is properly applied and uniform. Once the chemical cleaning is completed, a rinse-off process follows. Adhering to international standards also when it comes to the preparation of the steel surface for coating is essential. Accredited plants apply quality-control measures to ensure that the surfaces are prepared according to the required standards.
How The Coating Is Applied
The steel is hot-dip galvanised in a bath of molten zinc that has been heated to a high temperature. Once done, the steel is removed and set to cool down at the appropriate rate. Once cooling has taken place, the zinc completely bonds to the underlying surface.
The process is necessary to ensure the integrity of steel and to prolong the usefulness, strength, and reliable performance of the material. If water or moisture comes into contact with steel, it causes corrosion. The corrosion takes the form of iron oxide. The steel surface becomes flaky and the corrosion affects the structural integrity of the steel. This is called rust. To prevent steel from rusting, it is important to protect it from getting into contact with electrolytes. The two main methods used to achieve this entail the creation of an effective barrier between the steel and the electrolytes, and the creation of a protective layer of an anode that sacrifices itself. The latter is known as cathodic protection.
Zinc is used as the anode layer on the steel surface. For it to be effective, the thickness of the coat must be correct, the visual appearance must be just right, adhesion optimal, and application uniform. These factors are important for rendering high quality galvanised steel products. The thickness of the sacrificial anode layer determines how much protection the underlying steel product has, how long the product can be used, and how well the product can hold up when used for the intended purpose.
Quality is thus directly related to the correct application of the zinc coating. SANS 121- and ISO 1461-accredited galvanising plants thus ensure that the application meets the applicable requirements and thereby can guarantee quality steel products for applications in the construction, road-building, mining, infrastructure development, and engineering industries. Galvanisation extends the service life of the steel and thus ensures good cost-effectiveness of the steel products. Here are a few of the top reasons for the widespread usage of galvanised steel in infrastructure development, agricultural projects, mining, road construction, and engineering projects:
- Longevity. Galvanised steel products last for at least 25 to more than 50 years. With many steel products used outdoors, it is essential to protect against oxidation because of water coming into contact with the raw steel surface. If it is unprotected, the steel erodes within months. The environment in which they are used has an effect on the lifespan of steel products. Quality galvanisation helps to ensure the longevity of the products and thus reduced costs in terms of maintenance and/or replacement of materials. Maintenance is also straightforward, helping to reduce the cost over the entire lifespan of the product even further.
- Cost-effective when compared to other types of coatings. Although steel can be painted to protect it from the environment and elements, it is a labour-intensive, time-consuming, and expensive process. It also requires regular maintenance that adds to the cost of the item over its lifetime. With galvanised products, quality is guaranteed because the products are coated and tested according to relevant SANS and ISO standards. With the materials already galvanised when delivered to the project sites, installation can be done without having to paint or prepare the steel on site. As such, faster installation is possible. This is especially important with roadworks, where installation of, for instance, guardrails must be done in a timely manner as to minimise the risk of accidents because of ongoing roadworks.
- Reliable performance. The coating process is straightforward, minimising the risk of mistakes if done according to a proper quality-management system that is in compliance with ISO 9001. With the thickness or weight of the coatings specified, quality control can take place to precisely measure the applied thickness. The process thus allows for predictable and measurable results. ISO 1461 and SANS 121 provide the specifications and methods to measure the coatings.
- Fast turnaround. The hot-dip processes are conducted in accordance with the specifications of the relevant standards. As such, streamlined operations make it possible to apply coats within hours, as opposed to paint coats that can take up to seven days to complete before they protect the underlying material.
- Robust coatings. The zinc coat bonds metallurgically to the underlying surface because of the hot-dip process and cooling. Rather than the zinc simply being a coat over the steel, it becomes part of the steel. This makes the coat exceptionally strong. With it being robust, the protection is not easily damaged, as can be the case with paint when an item is transported and installed. This is an important consideration in the selection of steel products for construction projects. Materials are often transported in bulk to sites and even with the best care, paint coats can be damaged, exposing the underlying steel surfaces to environment and elements.
- Zinc protection of the entire product. The hot-dip galvanisation process entails the dipping of the material in a bath of hot, molten zinc. The zinc covers every part of the steel item. Because the zinc is in liquid form, it penetrates deeply into every corner and crevice. This ensures that no areas are left uncovered. The coating is often thicker at the corners where extra protection is required, as opposed to painted or sprayed protection that often thins out over the edges.
- Complete protection. The steel is protected in the sense that the zinc coat slowly wears off. This can take years, implying long-term protection. The coating erodes, rather than the steel. As such, even if the surface is scratched, the zinc is sacrificed, instead of the steel being eroded. If the damaged area is larger, the zinc sacrifice prevents rust sprawling sideways, as is often the case with paint coatings.
- Predictable quality control. The inspection process is straightforward and accredited plants have well-documented procedures for measuring the thicknesses of applications. Galvanised steel products can be used directly without first having to prepare the products on site. Buying such products from accredited plants thus means having the assurance of quality steel that will last for years.
In conclusion, hot-dip galvanisation helps to create stronger, long-lasting, and ready-to-use steel products for construction, roadwork, infrastructure development, and engineering projects.