While most people will agree stainless steel renders a long-lasting solution for many corrosion applications, the perception remains that stainless steel is expensive.
This is not true, even if the kilogram price of stainless is higher than other metals. This is because the price driver in the case of stainless steel is the alloying content of the material.
Stainless steel is an alloy of mainly iron and chrome with small amounts of other elements that enhance specific properties of stainless steels. By varying the content of these elements, different grades are developed for different applications. These elements give rise to the fact that stainless steel is not just one material, but rather a family of steels, each with specific properties.
A good understanding of the role of the chemical elements in stainless steel will give a good understanding of the different grades, their strengths, and limitations.
By selecting the correct grade for the task at hand, we can limit not only the initial costs but also the cost of maintenance and associated downtimes. Once this has been done, a cost analysis over the life span of the product will quickly indicate that stainless might be the material with a high initial cost but one which remains virtually unbeatable when it comes to total life cycle costs.
While this is advantageous, it still does not explain how to select the optimal grade of material for a specific requirement. To make the best material choices, it is also important to understand:
- The physical environment for the installation
- The functional application of the components
- Understand the nature of corrosion
Physical environment
Stainless steel is one of the most durable materials used in architecture and other external applications. Evaluating the environment can be critical in making correct grade selection. As mentioned, stainless steel is not one material, but rather a family of many different types with different properties and, most importantly, different levels of corrosion resistance.
If an appropriate stainless steel surface design and finish are selected and properly maintained, its appearance will remain virtually unchanged over the lifetime of the building, even if that life is well over 100 years. Alternatively, should inappropriate stainless steel and/or finish be used, corrosion can become a problem.
Sassda relies on the evaluation solutions offered by the International Molybdenum Association (IMOA) which is determined by European Standards (EN) and is now in the process of developing a unique product solution that will be suitable for South African conditions.
The evaluation system has five sections in which the following aspects of the environment is rated through a points system:
- Environment: The environment is rated as rural or according to various levels of industrial pollution.
- Coastal or Marine salt exposure: Chlorides are responsible for most environmental corrosion attacks. The level of salt exposure will be rated with a points system compiled in conjunction with the CSIR Corrosion map for South Africa.
- Local weather pattern: Local weather patterns determine temperatures and humidity levels which influence corrosion rates. It will also give information on rainfall which plays a role in “washing” the material.
- Design considerations: This includes factors such as surface roughness, finish grain orientation, sheltered or horizontal surfaces and crevices.
- Maintenance schedule: If stainless steel is susceptible to corrosion by salt (chlorides) or pollution, it must remain on the surface of the stainless steel long enough and in sufficient concentration for corrosion to start. Scheduled cleaning is required depending on the above factors.
These site evaluation tools are available from Sassda and the association can also assist in evaluating the corrosion potential for a specific geographic location.
The functional application of design components
Whilst the physical environment will determine the most cost-effective grade to cope with an environmental corrosive attack, the functional nature of the application can contribute to additional risks.
It is important to understand the process parameters to determine the risk for specific forms of either dry or aqueous corrosion. Once again design considerations and procedures for use and maintenance can play a critical role in optimising the life span of stainless steel equipment. The corrosion risk analysis will include information such as:
- The presence of process-related corrodents
- Product pH value, product solution strength, as well as operating temperatures
- The presence of chlorides and chlorine
- Design, fabrication, and operational issues
Sassda offers regular training webinars and courses aimed at educating members and industry on these matters and can assist with this type of operational evaluation.
Understanding corrosion
A good understanding of the possible corrosion mechanisms that are present in the environment and the process is required to recognised the potential risks and address them in time.
The mechanisms of corrosion are complicated and usually create conditions for other mechanisms to initiate. In many cases, the visible corrosion only acts as a symptom for different root causes. Identifying the root cause for corrosion can be material, design or process-related and can only be rectified after proper investigation and analysis. Sassda supports this process with corrosion investigation and mitigation.
Prevention is better than cure
A prior understanding of the environment, the process and corrosion parameters are required to make the best choice of material grade to deliver a cost-effective product.
It is essential to be able to evaluate failures and to learn from the experience. However, real cost efficiency cannot be achieved through failures. It is, therefore, significant to use cost-efficiency parameters during product development or the specification phase of a product or installation.
At first glance, it may look difficult to collate different combinations of initial costs, fabrication costs, maintenance, and downtime costs for different material choices when making the best economical decision in the project planning phase. However, a structured total lifecycle costing analysis can put all relevant material related costs over the entire lifespan of the project into perspective.
Sassda has developed a mobile App that calculates the total lifecycle costing for various materials. The result is provided in monetary value and gives users a complete and highly accurate analysis of material related costs.
By gaining material and corrosion knowledge and using the tools available on the Sassda platform, you can now ensure that the best economical decisions are made when selecting materials for critical applications. Simply Brilliant!