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Why Does Stainless Steel Corrode? - How-to Guide for Stainless Steel Use

Stainless steel bends

Does stainless steel corrode? Unfortunately, it does. Although stainless steel is known from high corrosion protection properties and is recommended for aggressive environments, it doesn’t give complete “immunity” to corrosion.  But, don’t worry stainless steel can rust under very specific conditions, e.g. contaminants enter the component during the manufacturing process, like welding. If You want to find out more causes for stainless steel corrosion and how to avoid them. Keep reading!

What is Stainless Steel and Why Use It?
Stainless steel contains mostly iron with a minimum of 10.5% chromium (acc. to EN 10020). Steel with a chromium content of at least 18% has been referred to as stainless steel. Additionally, austenitic steel features a 6-12,5% nickel content. Nickel is an austenite-forming element which facilitates plasticity forming and welding. When applied in a highly acidic environment, nickel slows down corrosion. Corrosion resistance is improved even further after molybdenum is added. A molybdenum additive is recommended in strongly acidic environments.
    
The natural corrosion resistance of stainless steel is due to the intrinsic, thin layer of chromium oxide which develops on its own on the surface and protects from corrosive agents. This layer rebuilds itself after the surface is subject to mechanical damage (chromium joins with oxygen), this process can take place when there is access to air, but also in water.

Stainless steel ducts
         

Stainless Steel Duct System
One of the reasons for the corrosion of the stainless steel component can be an error in the manufacturing process. What type of mistakes do we mean? For example, during production, the manufacturer needs to make sure that the stainless steel won’t be contaminated from contact with non-stainless steel elements, e.g. metal shavings, because these elements, even the smallest ones will corrode.

We recommend buying stainless steel ducting from reputable manufacturers. Choose a manufacturer that specializes in stainless steel and on a regular basis manufactures components made of stainless steel. Why? First of all, because such a manufacturer simply has stainless sheet coils in stock – ready for producing Your order.

Secondly, if You want to supply both stainless steel ducting and for example galvanized steel ducting from one supplier, a good manufacturer which uses mixed metals is aware of the risk of contamination. And the good manufacturer either has dedicated tools, machines for stainless steel manufacturing (segregation) or simply has cleaning procedure to avoid non-stainless steel contaminants.

Lastly, stainless steel is much harder material when compared to galvanized steel, therefore in order to make airtight stainless steel ducts, the manufacturer should use the best tubeformers (e.g. SPIRO International).

Stainles steel ductwork

Stainless steel is the second most popular material that we use for the production of duct systems (1st place takes galvanized steel, the 3rd goes to aluminium). We manufacture complete duct systems – both spiral and rectangular ductwork – made of the stainless steel as per AISI 304, EN-1.4301 standard with 18% chromium content and 9% nickel.

On request, we manufacture also ventilation components from the stainless sheet as per AISI 316L, EN-1.4404 standard which has an added 2-2,5% molybdenum content to improve the corrosion resistance in environments with high chloride content and in acidic environments (very often used in swimming pools, seaside environments, city centers).

How much does stainless steel ducting cost? Well, when compared to galvanized steel duct components, stainless steel is more expensive, approximately 5 to 8 times more expensive than standard galvanized.
Need an inquiry for stainless steel ducting – write to us: export@alnor.com.pl

Good Practices for Stainless Steel Design
It is very essential to take into account the requirement to periodically clean the acid-proof steel components at the design stage. A common mistake which hinders the future use of the installation is to design the installation, e.g. directly above the swimming pool surface. 

The installation should be cleaned regularly, and in the above-mentioned example, such maintenance is both difficult and expensive. Another important feature is the design itself as well as the method of joining steel which limits the risk of permeation and retention of the aggressive fluid or deposits which will never be flushed or removed. 

Remember to consider the type of working environment where the ventilation system is to be installed - in the highly aggressive, corrosive environments as well as with limited access to the ducting, You must take into account the probable loss of weight and use thicker components (piping for liquids with a high content of acidic or alkaline substances). Every acid-proof steel employed in difficult conditions will feature a weight loss eventually, as it is impossible to clean or rebuild the protective layer. 

6 Reasons Your Stainless Steel Ducting Corrodes (which make the manufacturer’s warranty void):

  1. No regular maintenance which leads to highly concentrated acids to form on the pipe walls (the water evaporates and leaves only the acid solution). 
  2. Incorrect selection of the stainless steel type, e.g. the steel type is not suited for so highly aggressive environments.
  3. The design does not allow for easy maintenance.
  4. Contamination during installation and preparation (lime, cement, foreign metals due to angle grinders being used in the vicinity or when improper tools are used during installation).
  5. Contact with regular carbon steel (damages during transportation or storage).
  6. Inadequate ventilation or no ventilation in an aggressive environment. Free airflow in the air duct is required at all times. 

How to Maintain Stainless Steel
To enjoy the long life and good appearance of stainless steel, it is important to see to its proper use and maintenance. This is especially relevant for locations where no constant cleaning with a neutral liquid is available. In standard climate, rainfall is sufficient to rinse highly aggressive chemical compounds from the metal surface. A common example can be seen in buildings located near the seaside, which are subjected to highly acidic environments (saltwater). In such buildings, places which cannot be rinsed by rainwater begin to corrode and lose their aesthetics. Walls and roofs which are regularly rinsed with rainwater are not damaged and maintain a good appearance for dozens of years. Cleaning removes deposits and dirt, which may cause corrosion and discolouration.    

Ventilation ducts, fittings and accessories manufactured by Alnor Systemy Wentylacji from materials indicated in the project documentation are always delivered clean and properly preserved. Components to be installed outdoors with low air pollution and periodically rinsed with rainwater require virtually no maintenance. However, most ventilation ducts are installed under the roofing and thus require regular washing and cleaning. To keep the manufacturer's warranty conditions, the following cleaning and maintenance guidelines are to be observed.

How Often Should You Clean Stainless Steel 
Recommended cleaning interval for ductwork made of stainless steel type 316L, for a clean environment, is once in 12 months, for highly contaminated environments e.g.: city centres, industrial centres, swimming pools, once in 6 months.        

For stainless steel type 304, the recommended cleaning interval is - in clean environments, once in about 6 months, for contaminated environments, every 3 months. Of course, this type of steel is unsuitable for highly contaminated industrial areas and, for example, swimming pools. 

What to Use for Cleaning and Maintenance?
For cleaning, we recommend regular warm water with soap. You can also use mild detergents, however, first test them thoroughly on a small surface. To improve the appearance of stainless steel, use the commonly available oil-based agents.

Whenever discolouration has set in and regular cleaning doesn’t bring any good results, use special agents based on a weak solution of oxalic acid or orthophosphoric acid. After you finish cleaning with any type of chemical agent, keep in mind to rinse the cleaned surface with lots of water to enable the invisible, protective layer of chromium oxide to rebuild.            

Ready-made water-based gels are available on the market, they allow to easily remove mineral calcium deposits, oil stains and foreign metal rust tarnish from all types of stainless steel surfaces. These agents are also antistatic. They are also silicon-free, which is especially important for ventilation systems.

OHS Guidelines    
Before attempting to clean, be sure to read the material safety data sheets for cleaning agents that are to be used and observe all the manufacturer’s recommendations therein. Use personal protection measures, ensure proper room ventilation and watch for fire safety hazards.

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Alnor Knowledge Base
Guidelines for using acid-proof steel components
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