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Stainless Steel in Rainwater Tanks

In our last article, we compared Poly Water Tanks with Stainless Steel Tanks. In this article we look further into the features of stainless steel in rainwater tanks.

About Stainless Steel

Unlike carbon steel, stainless steel will not readily corrode, rust or stain from water contact and moisture. Stainless steel is also known as inox steel, derived from the French word “inoxydable”.

The strength and durability of stainless steel makes it a desirable yet expensive option. Due to its expense it is normally used in products where both the strength of steel and corrosion resistance are required. So we see stainless steel used in products like kitchen sinks, cooking vats or rainwater tanks.

The main difference between stainless steel and carbon steel is the amount of chromium present. Chromium is a chemical element that is a hard, steely-gray and lustrous metal. It protects stainless steel by forming a chromium oxide layer. This layer provides a protective shell that blocks oxygen seeping into the metal’s internal structure which would cause corrosion.

Is Stainless Steel Truly Stainless?

While stainless steel in theory should not corrode, we have all probably notice rust forming on a “stainless” steel product. In lower quality stainless steel product, or a poorly made stainless steel products, it could be that the chromium levels are not high enough or evenly distributed throughout the stainless steel.

Stainless steel generally contains between 12-30% of chromium. Chromium protects stainless steel as long as the localised concentration is in excess of 12%. If the stainless surface is contaminated by other metals or chemicals like chlorine, then the localised concentration of chromium can fall below 12%. This then results in the chromium oxide barrier failing and what is known as “pitting” to occur.

So what external factors might negatively affect a stainless steel surface and cause it to rust?

The simplest condition under which rusting occurs on stainless steel is when ordinary steel is rubbed off onto a stainless steel surface. This leaves a shallow film of iron which contaminates the stainless steel surface. The iron rusts, and the corrosion will stop provided the stainless steel surface remains exposed to the atmosphere. While the rusted surface looks bad, the chromium in the stainless steel forms a corrosion-resistant oxide layer once the film of unalloyed steel rusts out.

Another scenario could be if the stainless steel comes into contact with chloride (found in salty sea water), or chlorine (found in swimming pools). These chemicals disrupt the normally protective chromium oxide layer allowing corrosion to occur.

Sadly, such contamination can cause pitting to occur in the stainless steel. Pitting is a localised form of corrosion that can lead to small holes or “pits” in the stainless metal. It is therefore important to regularly clean stainless steel products and equipment with fresh water. If you notice rust spots occurring on your stainless steel product, then remove the rust spots and clean as soon as possible to prevent irreversible pitting.

Stainless Steel in Water Tanks

While stainless steel is an expensive material that should not rust or corrode, it is perhaps more correct to refer to stainless steel more as ‘highly stain resistant’ (for reasons previously stated such as pitting). For this reason, warranties will often state that your stainless steel tank:

  • must not being installed in wet concrete or soil
  • must have any visible staining cleaned off (as contaminants as discussed above can cause more permanent staining in the form of pitting corrosion).

Since external contaminants and a lack of surface exposure to the air can make the chromium oxide protective barrier fail, stainless steel tanks are not suitable for underground installation. You will not find a stainless steel underground tank model.

Depending upon the grade of stainless steel used in your rainwater tanks, some warranties will not cover marine environments. Generally you will want your water tank to be a minimum grade of 304 Stainless Steel, which is the same as your kitchen sink. Obviously, kitchen sinks are very durable. For marine environments it is recommended that you obtain 316 Stainless Steel.

While stainless steel is obviously durable, some who buy them end up with leaks due to corrosion of the joints, rivets or screws. These have the potential to be the source of corrosion and leakage. If this happened, then it obviously renders your rainwater tank useless and a greater expense to fix.

Solders can also greatly affect the safety of your rainwater. An investigation by the Public and Environment Health Services in Tasmania found water in some stainless steel tanks containing harmful levels of lead. Lead was used in the solders which then corroded and leaked into the water. Sadly, lead is very toxic to our bodies and is especially harmful to the development of young children.

To overcome these problem,s spot welding can be used for joining the stainless steel sheets. Keep in mind that welding can also release traces of iron-rich material that then rusts. It is therefore important that you do not just get anyone to do this or attempt do it yourself.

For a comparison of stainless steel water tanks versus poly water tanks, read Water Tanks Compared – Poly Tanks versus Stainless Steel Tanks.