Why stainless steel in and around the swimming pool?

You will find stainless steel somewhere in any swimming pool – even if it is only a few screws and nuts. Though they can be made of other materials these days, like plastic, stainless steel still has its domains:

  • Firmness and stability
  • No or hardly any signs of wearout
  • Less corrosive than other metals, like aluminium, copper, or brass
  • Optical attractiveness through a polished, shining surface
  • Environmentally friendly, as it is extremely long-lasting and 100 per cent recyclable
  • Resource friendly, as a very high percentage of scrap metal is processed
  • Easy to clean, especially in public pools
  • More hygienic than alternative materials
  • Frost-protected in outdoor pools

Stainless steel materials in the swimming pool area, and their cue marks

From the wide-ranged family of stainless steels, with relationships that are very hard to understand for the average layman, two groups have proved to be very reliable for swimming pools accessories all over the world.

Material 1.4301 (a.k.a. 304)

  • Used to be the swimming pool industry’s favourite material for stainless steel accessories.
  • 304, like all materials described hereafter, belongs to the so-called austenitic steels, meaning it is not magnetic. Thus, you can easily verify if you really have 304 material by using a simple magnet. But please keep in mind that in bent areas, the material might prove very slightly magnetic. After all, the main material –some 70 per cent – is iron.
  • The alloy is crucial for the respective firmness of each stainless steel. The 304 alloy consists of some 18 per cent of chromium and 10 per cent of nickel, and also max. 0.07 per cent of carbon, which is caused by the production process. Only chromium and nickel are important as far as firmness is concerned.
  • 304 can be used in swimming pools with a completely normal and unobtrusive water quality. Yet something this material does not like at all are chloride concentrations exceeding 150 – 200 mg/l. However, as in many German regions, these concentrations occur in normal tap water – which, of course, is used to fill a swimming pool – another stainless steel material has become increasingly popular in the past few years.

2.2 Material 1.4401/1.4404/1.4571 (a.k.a. 316)

  • These materials have a superior alloy compared to 304, and are therefore more resistant against chloride attacks.
  • Its tolerance limit is between 400 and 500 mg/l. 316 contains 16.5 – 18.5 per cent of chromium, 10 – 13 per cent of nickel, and, additionally, 2.0 – 2.5 per cent of molybdenum.
  • Unfortunately, the alloy raises the price for this material above that for 304.
  • The price difference is about 40 per cent for the raw material.
  • Basically, both 304 and 316 are available as sheet and tube metal in the market without any problem.
  • 316 is most commonly used in public and private swimming pools. The steel resists even difficult water and atmospheric conditions.

2.3 Materials 1.4539 and 1.4462

  • Salt water and thermal swimming pools are getting increasingly popular. Chloride concentrations of way more than 3,000 mg/l, along with water temperatures exceeding 30°C, are common and may give the stainless steel a hard time. Constant surface cleaning might be a way to preserve the steel stability – but this may prove to be a problem when parts are mounted below the water surface. For these areas, there are tendencies to use materials even higher alloyed, like 1.4539 or 1.4462, whose advantage is their excellent chloride resistance.
  • In the market they are, however, mostly unavailable in many shapes and measurements, very hard to process with normal machines due to their extreme toughness – and, above all, outstandingly expensive.

Stainless steel corrosion

After deciding to have a certain material, and after receiving and mounting a beautiful, gleaming stainless steel ladder into his swimming pool, the pool owner will most certainly not be pleased to detect brown discolorations on the ladder or other parts after a rather short time. In this context, the term „stainless steel can’t rust“ is frequently used. This may suggest to the customer that either the material or the processing is faulty. Although manufacturing certainly cannot be avoided completely, they are still very, very rare. Even if your customer may not like it – in most cases, the water quality or external influences have caused the discoloration or corrosion. As we have already said above, even „stain“less steel consists mostly of iron and is not really non-corrosive but, to a certain extent, resistant to corrosion.

This resistance is caused by an oxide layer which is formed with the help of the chromium on the steel surface. This surface, called passive layer, protects thesteel permanently from corrosion. If this passive layer ist destroyed, the steel loses its resistance.

Reasons for corrosion from swimming pool experience

Wrong dosage adjustment in the automatic chlorine proportioner

Even fully automatic chlorine proportioners do not function completely faultlessly, and should be subjected to regular maintenance.

Drilled armour mats

Concrete pools are armed with steel mats. It happens so that during the installation of ladders or handrails, the rods get accidentally drilled into. The consequence is rust, which pours from the drillhole and settles on the screws.

Insufficient fresh water supply

It is appropriate to be skeptical if the swimming pool owner proudly tells the following: „Look at the water – it’s crystal clear, even though we have not changed the water in 12 years.“ Only that your ladder has been rusty since last year. And under the circumstances given, that’s no wonder: for the chloride concentration can only be decreased by adding fresh water. Otherwise, chlorides and many other water chemicals cannot leave the pool water. So, you can imagine what a cute little chemical cocktail the (now not quite so proud) pool owner has got in his water. The pool water should be changed completely at least once a year.

Chlorine supply in the immediate vicinity to stainless steel parts

If the pool owner does not leave the water care up to the automatic chlorine proportioner but adds chlorine himself, he must take care not to put the chlorine tablet (or powder) directly onto the ladder step. Upon dissolution, there are chloride concentrations exceeding the tolerances by far.

Injury through base metals

If a stainless steel screw is fixed with a tool which, for example, fixed some plain steel screw before, minuscule particles of the plain steel screw can be transferred to the stainless steel screw, and lead to contact corrosion on the stainless steel screw head.

Corroded piping

In modern piping, mainly plastic or corrosion proof materials are used. However, people did not consider them worth their while in the 60’s and early 70’s. Therefore, even when a pool is filled for the very first time through such pipes, rust particles can be transported into it. The rust searches and finds the cool stainless steel surface, settles there, and starts working soon, which means it destroys the passive layer.

Missing or faulty welding work

Stainless steel parts are often welded together. It is important that the welding residues, like blue film, cinder, and tarnishing are removed from the steel surface. This is accomplished by pickling. Stainless steel etchants are available as liquids or pastes. The pickle removes a tiny part of the surface, and the contamination also. Of course, this also destroys the protective passive layer on the stainless steel, but this is able to reconstruct itself in a matter of hours without any assistance. If the pickling is not conducted efficiently, the remaining welding residues bloom in the aggressive pool water.

Incorrect cleaning

Incorrect cleaning is, besides the chlorides, the main reason for problems with stainless steel parts. The expert easily recognises the corrosion reasons rather easily. Instead of shining metal, a deep rust-red is the basic colour of the entire stainless steel surface. When asked, the pool owner often claims that only detergents permitted for swimming pools have been used. However, even highly acidiferous tile cleaning agents are permitted – which are absolutely not suitable for stainless steel.

Stainless steel care

Though stainless steel care is an important point, many pool owners more or less ignore it.

Compare it with the stainless steel sink in your kitchen: How often do you clean and polish that one? Daily, or at least several times a week! But the stainless steel handrail in your swimming pool is fitted and forgotten.

If the water quality is not problematic, it is certainly sufficient to take care of the stainless steel parts twice a year.

The problem arises when pools are run at high temperatures, the pH value is too low, or salt water is used. Here the parts must be taken care of more often, at least once a month.

As already mentioned above, not all detergents are equally suitable for stainless steel. Slight lime residues, or the first signs of discolouration, can be removed with a normal stainless steel cleaning agent, the kind you use for your kitchen sink. Please rub with a soft cloth only, never with steel wool or the like! While reliably removing any residue, it would also rub plain steel particles into the stainless steel surface, which start corroding two weeks later. For dirtier parts, and for the first, harmless, corrosion spots, please use the slightly acidiferous, special stainless steel cleaning agents available from your swimming pool dealer. Please read the instructions carefully before use, and rinse with lots of fresh water afterwards. If, however, the stainless steel part happens to be badly corroded, the only possibility of saving it is to dismantle it and send it to the manufacturer for reprocessing.

How to measure the chloride value?

As automatic chlorine proportioners do not measure the chloride value, the chlorine values are often equalled or confused with the chloride value.

There is a very simple and quick method to measure the chloride value, though. By using the chloride tablet counting method, the chloride value can be detemined within a few minutes, as precisely as 100 mg/l. Simply take a water sample and add tablets until the water colour changes from yellow to brown. Count the tablets added, subtract one, and multiply the remainder x 100.

The sample volume (10ml) changes its colour after 7 tablets. (7 – 1) x 100 = 600mg/l chloride content.