The general perception of Stainless Steel is that the material maintains a corrosion free and a stain free surface, whatever the conditions or environment. This perception is sadly not the case.
What is Stainless Steel ?
To understand the metallic alloy known as Stainless Steel and its many forms, characteristics and properties, we firstly to understand the definition of the metallic alloy called “Steel”.
The basic definition of “Steel” is simply an alloy of Iron and carbon. The non-metallic chemical element Carbon being introduced at the manufacturing stage, to bond the Iron particles together and thus making a stronger, more workable metallic alloy that basically revolutionised the engineering and manufacturing industry.
The downside of Steel in its crudest form, as described in the last paragraph, is that it corrodes very easily in an oxidising atmosphere, where the oxygen combines with the Iron (FE) to form Ferric oxide, commonly known as Rust.
In the early 1900’s, Metallurgist’s strived to add and exchange the Iron (FE) particles for other more corrosion resistant metallic particles like Chromium and Nickel. Also, other non-metallic particles were used to replace Carbon as the bonding particles, such as Silicon, Sulphur, etc. These Newly formed alloys were named firstly as “Rustless Steels” and a few years later, became known as Stainless Steel.
The metallurgists did not stop there and carried on with the advancements of this new range of metallic alloys, by controlling cooling times from the molten stage and by adding other, more exotic materials such as Molybdenum, Titanium, Manganese, etc.
Today, there are thousands of different grades of Stainless Steels. Some of which are highly corrosive resistant and some of which are less corrosive resistant, but high in wear resistance. All these different grades have different, but unique properties and many have been alloyed for specific purposes, such as extreme cold environments, such as North Sea applications.
For Swimming pool manufacture, the design engineer is governed by a number of factors such as (a) The commercial availability of material in sheet form, tube form, etc, and (b) The corrosion resistance, weldability, durability and longevity of the material. With these requirements in mind and without going into complex metallurgical explanations of the different families of Stainless Steel, the Design Engineer will conclude his choices to within a few grades of alloys within the Austenitic Family of Stainless Steels. Namely, grades 304 and 316 or for weldability, their variations with a low controlled carbon content, grades 304L or 316L. (NB: Duplex stainless steels are sometimes chosen where elevated heat is introduced to the working environment, such as swimming pool heat exchangers, etc).
Having a brief understanding of the importance of correct material choice, we now need to understand how the corrosion resistance plays its part in the necessity for adopting the correct maintenance methods to sustain that corrosion resistance through a simple after-care program.
The Protective layer of Stainless Steel
To explain this concept, please think of a brand new, state-of-the-art Ferrari Car. The car at its sale point is stunningly shiny red and its exterior lacquer gleams in the slightest ray of sun light. However impressive this is at its conception stage, the owner would still be required to maintain that pristine appearance by periodically cleaning and waxing the paintwork to resist any flaws or deterioration of the paint lustre.
Moving this concept to Metallic elements. All metals have an external protective layer called the Oxide Layer (Or Passive Layer). This exterior, naturally forming layer, is the metal combining with oxygen to protect itself from corrosion and the surrounding environment. If you can imagine that Copper goes green, Brass dulls quite quickly and so does Aluminium. This is caused by the self-forming protective Oxide Layer. This Oxide Layer of Stainless Steel is totally transparent.
Therefore, Stainless Steel will continue to be “Stainless”, providing the Oxide Layer remains intact.
So now that we understand the importance of the Oxide Layer, how can this be (a) removed or destroyed, and (b) be restored:
(a) The oxide layer is a very shallow chemically reacted film on the outer surface of the material and can easily be removed by light abrasion, welding or certain forms of chemical attack. Removal of the Oxide layer, exposes the bare metal to the surrounding atmosphere and where that atmosphere is corrosive, degradation will quickly become apparent.
(b) The oxide layer will automatically restore over time and this time differs from metal to metal and also is dependent on the storage environment, room temperature, etc. For Stainless Steel, this self-formation could take one to two months. During this time, the metal surface is susceptible to attack and deterioration can quickly take place. Therefore, if the Oxide Layer is removed, it will need to be re-formed as quickly as possible, the method of restoring the Oxide Layer by a creating a simple chemical reaction, is called PASSIVATION.
Maintaining the surfaces of Stainless Steel
Now we understand Stainless Steel a little better and understand the concept of the metallic protection layer, namely The Oxide Layer. We can apply this knowledge to the daily maintenance of our Stainless Steel investment. In this case, A Swimming Pool and associated ancillary equipment.
So, we have assumed that the design engineer has selected the correct grade of stainless steel and we will assume that the manufacture has treated all the surfaces, including welded detail, by chemically passivating. These should be apparent in the OEM Manual by way of material certification and Certification to treatment or Process conformity.
Having assumed the above and bearing in mind that the Stainless Steel structure and ancillary equipment are about to be subjected to a highly corrosive environment of the presence of Halogens chemical elements (Chlorine, Florine, etc) and an atmosphere of high humidity. We now have to be extremely diligent in the manner that we routinely maintain and carry out any repair work that can be detrimental to our hugely important protective Oxide Layer.
This is where we conclude with the simile to the Ferrari Car syndrome, or any other car for that matter. We must keep our surfaces clean by periodic “soft” cleaning. That means absolutely NO ABRASION whatsoever. We can only use abrasion in the occasions of heavy rusting, but must be extremely careful to follow a strict cleaning procedure with re-passivation of the material surface,
If we follow the correct periodic cleaning method and if no other metallic components have been in contact with our structure, then no heavy corrosion removal will ever be needed.
In the event of and necessity of corrosion removal, we should always carry this out by a safe method of dissolution.
The following procedure should be adhered to for:
1. Periodic maintenance cleaning (Monthly) – Soft cleaning
2. Removal of Corrosion (If necessary) – Rust removal – hard cleaning
Please see attached cleaning procedures and please ensure that all precautions are taken and the data sheets are fully understood.
This report has been compiled by:
C J Owen BSc (Hons)