30934 V2
R Series
Page 15
Operating Manual
www.grantinstruments.com
7.0 Technical Tips
7.1
Which water should you use in your R unit tank?
For the long-term reliability of the equipment it is important to use oxygenated water that
is free from ions and minerals that can cause corrosion of stainless steel. We
recommend the use of distilled water and de-ionised water from modern ion exchange
systems that do not use salt back flushing to regenerate the ion-exchange cartridges.
Stainless steel is protected from corrosion by a layer of chromium oxide. If the layer is
damaged, oxygen present in water can reform the oxide layer. If the water is still or de-
oxygenated, and the oxide layer is damaged, ions can corrode the stainless steel tank. If
a water bath has been unused for some time, or water boiled, we recommend changing
to fresh distilled water or correct de-ionised water.
Water normally contains calcium or magnesium ions. De-ionised water has most ions
removed as indicated by its conductivity level; the purer the water the lower the
conductivity. It is important to use only de-ionised water from an ion exchange system
with replaceable cartridges. Do not use de-ionised water generated from an ion-
exchange system that incorporates a salt back-flush system to regenerate the ion-
exchange resin as this can leave sodium ions that are very corrosive to stainless steel.
7.2
How to prevent rust in R unit tanks
Most Grant tanks, as well as immersed parts, are made from type 304 stainless steel, an
extremely versatile general purpose grade of stainless steel. It is the excellent forming
characteristic that has made this grade dominant in the manufacture of laboratory and
industrial water baths, as well as domestic sinks and saucepans. Type 304 stainless
steel is highly suitable for applications where hygiene is important; it exhibits good heat
resistance and excellent resistance to corrosion.
However, despite resistance to general surface corrosion, stainless steel is susceptible to
specific types of corrosion, in particular pitting (small pin hole style corrosion) and stress
corrosion cracking. It can also undergo general corrosion in specific environments, such
as one containing hydrochloric or sulphuric acids.
Stainless steel is protected by its high content of alloying elements, primarily chromium
and nickel. Chromium is the most important with respect to corrosion resistance,
although the nickel assists in allowing the chromium to do its job. The chromium forms an
oxide layer on the surface of the steel, which inhibits further oxidation. This layer adheres
extremely well to the metal substrate, but it is essential that it remains intact, and must be
protected from various forms of damage.
If the surface chromium oxide layer becomes damaged, oxygen present in water can
partially reform the oxide layer, so it is advisable to ensure that water is always fresh and
well oxygenated. Baths that will be out of use for an extended period should be emptied,
and all moisture should be wiped from the bottom of the tank.
In some cases a brown layer may appear on the surface of a stainless steel tank. In most
of these cases this is not rust, but it may be a surface deposit of minerals from the local
water supply, or ferrous particles or salts that have fallen into the tank. These surface
deposits can usually be removed by using a household cleaner such as Duraglit or Silvo
metal polish.