The general perception of Stainless Steel is that it is totally resistant to staining and is indeed “stain-less” in all application. However, this is not the case.
This article attempts to explain in broad terms, the importance of choosing the correct stainless steel, how to ensure that the stainless steel is prepared correctly for application and also, how to remedy a product that has been stained or corroded, if not treated correctly.
What is Stainless Steel?
Stainless Steel is a generic name for a family of corrosion resistant metallic alloys that contain more than 11% Chromium. To avoid the “stainless” confusion, a more appropriate name for this group of materials would be “Corrosion resistant steels” that have varying properties, depending on the alloying elements and methods of production.
Specifying the correct Stainless Steel
Before making the final decision on which grade of stainless steel you should use. It is important to consider what type of application it is being used for.
When considering stainless the most important features are: -
- Corrosion resistance
- Mechanical & physical properties
- Available forming, fabrication & joining techniques
- Environmental, sustainability & material costs
Having decided on the material application, there are four major types of commercially available stainless steels ranges to choose from, namely:
- Austenitic. Typically known as the 300 series and are the most widely used type of stainless steel. The materials in this range will have a Chromium content of at least 16% and a nickel content of at least 7%, which makes it ductile, corrosion resistant and allowing excellent weldability properties. Applications would be pharmaceutical products, food and beverages manufacture, marine industry and external architectural structures.
- Ferritic. This material has similar mechanical properties to Mild Steel, but offers a greater corrosion resistance than that of Mild Steel. The Chromium content can vary between 12% and 17% and typical applications would be house wares, automotive exhaust systems and internal architectural structures.
- Martensitic. This material has been produced to form a complex face centre cubic lattice called “martensite” which renders the material hard and brittle. The material can be heat treated to vary its toughness and mechanical properties. Typical applications would be knife blades, surgical instruments and wear resistant castings.
- Duplex. This material has a combined structure of ferrite and Austenite and has a high resistance to corrosion, equal to the Austenitic range and will also resist stress corrosion cracking at elevated temperatures. Typical applications are heat exchangers, water treatment plant and the petro-chemical equipment.
As mentioned previously, the most extensively used range of Stainless Steel is the Austenitic range. Within this range, the most common types used in exterior architectural structures are grades 316 and 304. bIt has long been recognised that the surface finish of Austenitic Stainless Steel has an important effect on its corrosion resistance.
However, by specifying a particular grade of Stainless Steel especially for exterior applications, is not in itself sufficient enough.
The surface protective layer of Austenitic Stainless Steel is actually an extremely thin but stable and passive Chromium rich oxide film, on which material relies for its corrosion resistance. If this oxide layer is not intact, the material will show signs of staining, corrosion and even surface pitting, in a very short space of time.
Mechanical processes such as grinding, polishing, welding and machining, will remove the protective surface oxide layer and expose the material to rapid acting air-borne contaminants and aggressive environments, such as sea fronts, marine and swimming pools. Consequently, the use of mechanical processes can actually accelerate a corrosion problem.
Therefore, It is imperative that you maintain the passive oxide layer to prevent corrosion and staining at all times.
The recognised method of chemically restoring the protective oxide layer is known as passivation. This process creates a spontaneous formation of a non-reactive surface film that inhibits corrosion.
There are various methods of passivating Austenitic Stainless steels; the most widely used being a 20 percent-by-volume Nitric acid solution. This is a highly corrosive substance and can only be used under controlled conditions.
To fully Passivate and to achieve the ultimate corrosion protected surface of an Austenic Stainless Steel, a pre-treatment of electropolishing is recommended. This process removes any free iron from the surface of the material and improves the surface finish, as well as enhancing the surface Chromium oxides. However, this process does create a bright chrome-like surface and is not always desirable for external applications.
Therefore, to specify an Austenitic Stainless Steel component or structure to withstand staining and corroding, the designer should consider the following:
- Does the application allow the material to be electropolished and passivated before installation. If so, this will ensure that no staining occurs for prolonged periods and reduce further maintenance costs and increase longevity of service.
- Does the application allow the material to be passivated before installation. This should be the minimum requirement if the brightness of electropolishingpolishing is not suitable. This will provide a very good corrosion resistant surface, but some periodic cleaning may be required in certain environments.
In a nutshell, many fabricators, manufacturers and workshops that use Stainless steel, do not fully appreciate the fundamental metallurgical requirements to achieve fit for purpose components and as a result, readily turn their backs on the resulting problems that occur from that lack of knowledge. The aftermath can be extremely costly, frustrating and sometimes very difficult to remedy after installation.
Once an Austenitic stainless steel installation takes place, it is immediately subjected to its surrounding environment and conditions. If at this point the protective oxide layer is not fully formed, staining and corrosion can quickly follow.
As mentioned previously, the widely used passivation fluid of Nitric acid is a very dangerous and toxic material. Thus, due to environmental laws and regulations, cannot safely be used to passivate a structure that as been installed or erected.
MPE Limited is a company that specialises in the processing and treatment of stainless steels and has created a range of chemical treatment products, called The Willowchem Range.
Two products within this range have been carefully formulated to provide a safe and lasting solution to on-site staining problems on Austenitic Stainless Steel.
The Willowchem 61 product has been designed to dissolve and clean surface contaminants and is perfectly safe to use. This will leave a sharp, clean, uniform, stain free surface that is free impurities.
The Willowchem Biopass product is a unique formulation with a powerful oxidising agent that passivates and disinfects the material surface. This product is non-hazardous and totally biodegradable and will leave the material surface in a fully passivated and protected condition.
MPE Ltd can offer on-site solutions and renovations to Stainless steel installations, as well as offering consultation reports and organising seminars in material awareness, material selection and CPD (continual professional development)
Written By: Colin Owen BSc (Hons)
MPE Ltd, Units 6 & 7 Hirwaun Industrial Estate, Hirwaun, Rhondda Cynon Taff, CF44 9YE.
Tel: 01685 812765