Technical Tips - VWR 462-0353 Instruction Manual

6

Technical Tips

6.1
Which water should you use in your bath?
For the long-term reliability of water baths 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-ionized 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-ionized water.
Water normally contains calcium or magnesium ions. De-ionized 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-ionized water from an ion exchange system with replaceable
cartridges. Do not use de-ionized 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.
6.2
How to prevent rust in water baths
Most VWR 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 or metal polish.
6.3
How to prevent algae and bacteria
Water baths provide the ideal environment for the growth of micro-organisms. If left
uncontrolled the growth of these organisms can result in a range of serious problems and
health risks from pathogenic bacteria.
VWR shaking water bath Instruction Manual
European edition
EN
31228 V1
Page 9