CPD: Materials for corrosion resistant and hygienic drainage
This CPD is sponsored by ACO Building Drainage. It aims to help specifiers gain knowledge allowing them to avoid specification risks associated with drainage elements for corrosion resistant and hygienic applications.
ACO Building Drainage specialises in the development of corrosion resistant drainage systems and building products for applications across the internal and external built environments. Conceived to eliminate design risk, to optimise installation and to minimise lifetime ownership cost, every product in the range delivers exceptional levels of performance, finish and durability.
The technical expertise, global resources and fabrication capacity within ACO Building Drainage also make it possible for architects, engineers and contractors to obtain the highest quality and value the ACO range of products.
Part 1: What is stainless steel?
A steel becomes stainless when at least 11 per cent chromium is added to the iron based alloy.
Stainless steel has now been around for more than 100 years. It was first produced by researchers developing corrosion resistant alloys and used to prevent rusting of gun barrels during the First World War.
Since then its durability and corrosion resistant properties have been widely utilised in engineering applications including aerospace, automobile, petro-chemical, pharmaceutical, medical, scientific, white goods, domestic, cutlery and construction products including drainage systems.
New York’s Chrysler building, built in the late 1920s and clad in stainless steel has successfully withstood acid rain and the corrosive city environment for more than 75 years. London’s Savoy Hotel’s façade is also manufactured from stainless steel and has been standing for a similar length of time.
Part 2: Stainless steel grades
There are over 150 different grades of stainless steel. These are categorised into four core families; ferritic, martensitic, duplex and austenitic. It is the austenitic grade of steel which is used for drainage products and the only family of stainless steels that are non-magnetic.
Magnetic grades of stainless steel are not appropriate for drainage elements as often minute iron particulates can be found in wastewater. If these particulates become magnetic they can be immobilised on to the surface of drainage elements and the following issues may arise:
- Natural galvanic corrosion of the drainage element due to the dissimilar metals in direct contact with each other in an aqueous environment
- Crevices can form adjacent to the particulates where pathogens can grow and multiply, creating cleaning issues for the operator and ultimately a threat to hygiene
Typical alloying content of the different stainless steel families – the balance of which is iron:
|Typical composition (% weight)|
|Stainless steel family||Popular notation||Carbon||Chromium||Nickel||Molybdenum|
Most building drainage applications use austenitic 304 grade steel, whereas for particularly hostile environments 316 grade may be required. The fundamental difference between grades 304 and 316 stainless steel is the addition of molybdenum of between 2 and 2.5 per cent. The raw material costs for 316 grade stainless steel increases by around 60 per cent compared to 304.
Part 3: Corrosion resistance
Stainless steel is naturally corrosion resistant. This resistance is formed by a thin, but tough chromium-rich oxide film.
Despite the name, stainless steel, can stain and corrode if it is used incorrectly. To avoid corrosion issues, the following will need to be considered:
- Exposure to reducting solutions, such as hydrochloric and sulphuric acids which need careful consideration
- Gasket and seal materials also need to be considered – neoprene and EPDM are generally adequate for most applications, but Viton may be needed for particularly hostile environments
In order to correctly specify the drainage system it is important to establish the cocktail of chemicals which the drainage system will be exposed to; their concentration; the temperature of the solution – this is most important and often forgotten as corrosion rates can dramatically increase as the temperature rises; and the amount of time which the drainage system will be exposed to the chemicals.
Part 4: Stainless steel fabrication and finishing processes
The fabrication process of cutting and forming will introduce impurities such as embedded iron into the surface of the stainless steel. If this is not removed then corrosion of these iron particles can occur causing pitting and rust spots. Additionally, the welding process destroys the all-important chromium-rich oxide film protecting the stainless steel from corrosion around the welded joints rendering the product particularly vulnerable to corrosion.
The full corrosion resistance of the fabricated product can be restored using the pickle passivation process which is a controlled corrosion process using nitric and hydrofluoric acids to remove any embedded iron particulates and restore the passivated layer. Pickle passivation is considered to be the ultimate post-fabrication process, restoring the chromium-rich oxide film to the stainless steel, and allows reliable prediction of corrosion resistance.
Electro polishing can also be used as a finishing process. This is an electrochemical process and is in essence the reverse of electroplating where a small amount of material is removed from the surface of the stainless steel. The resulting finish is a highly polished lustre, not dissimilar to chromium plating in appearance and generally applied for aesthetic purposes.
Part 5: Surface finish and hygienic performance
Stainless steel sheet is generally used for the fabrication of drainage elements.
2B stainless steel material is cold rolled and skin-passed with a light roll to improve the surface finish. The surface finish is key to its hygienic performance. The roughness of the surface ranges from 0.3 to 0.5µm Ra which is smaller than the geometry of Listeria bacteria and therefore allows easily cleaned drainage surfaces.
Listeria can kill humans, and in a survey carried out in 1990, which swabbed 10,000 high risk areas, it was found in 23 per cent of drains tested. Listeria is particularly difficult to control as it survives in a temperature range from -0.5°C to +45°C.
Providing drainage elements with rounded contours and the avoidance of lapped joints significantly improves hygiene performance in the prevention of bacteria traps.
Part 6: Galvanised steel
Galvanised steel can be used for drainage applications however it does have limitations in both corrosion resistant and hygienic applications.
For galvanised steel, zinc is applied to the base material to prevent rusting of the mild steel substrate. Products can be manufactured either from pre-galvanised sheet or by dipping the steel into molten zinc after fabrication to form hot-dipped galvanised steel.
Generally, galvanising is best suited to external storm water drainage applications. Stainless steel is significantly more appropriate for internal drainage use where high corrosion resistance and hygiene requirements are pre-requisites.