What could cause my liquid nitrogen dewar to rust?

Firstly its important to know what grade if stainless steel liquid nitrogen dewars are manufactured from. Then we can understand its properties and characteristics.

Dewars are specificallly designed for transportation of liquid nitrogen and are made from Type 304 stainless steel which is austenitic. This means that steel contains a form of iron called gamma phase and is combined with another alloying element. It has a minimum of 18% chromium and 8% nickel, combined with a maximum of 0.08% carbon. It is defined as a Chromiumm-Nickel austenitic alloy.

The austenitic stainless steels possess a unique combination of properties which makes them useful at cryogenic (very low) temperatures, as are encountered in plants handling liquefied gases. These steels at cryogenic temperatures have tensile strengths substantially higher than at ambient temperatures while their toughness is only slightly degraded

Other characteristics include:
– Forming and welding properties
– Corrosion/ oxidation resistance thanks to the chromium content
– Deep drawing quality
– Low temperature properties responding well to hardening by cold working
– Ease of cleaning, ease of fabrication, beauty of appearance

Although stainless steel is much more resistant to corrosion than ordinary carbon or alloy steels, in some circumstances it can corrode. It is ‘stain-less’ not ‘stain-impossible’. In normal atmospheric or water based environments, stainless steel will not corrode as demonstrated by domestic sink units, cutlery, saucepans and work-surfaces.

Pitting Corrosion – The passive layer on stainless steel can be attacked by certain chemical species. The chloride ion Cl- is the most common of these and is found in everyday materials such as salt and bleach. Pitting corrosion is avoided by making sure that stainless steel does not come into prolonged contact with harmful chemicals or by choosing a grade of steel which is more resistant to attack.

General Corrosion – With some chemicals, notably acids, the passive layer may be attacked uniformly depending on concentration and temperature and the metal loss is distributed over the entire surface of the steel. Hydrochloric acid and sulphuric acid at some concentrations are particular aggressive towards stainless steel.

Look after your dewar:

Do not store it in wet, dirty areas. Moisture, animal waste, chemicals, strong cleaning agents and other substances which could promote corrosion should be removed promptly. Use water or mild detergent for cleaning and dry the surface thoroughly. Do not use strong alkaline or acid cleaners that could damage the finish and corrode the metal shell.

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