Water treatment (Topic 11051)

In closed systems with or without circulation is it possible to decrease the corrosivity of the liquid by adding inhibitors, biocides or oxygen scavengers.

The experience with biocides and oxygen scavengers in connection with aluminium is very limited and these additions should be used with caution. Preferably the supplier should be able to provide the customer with data from aluminium systems where these chemicals have been used.

Several kinds of inhibitors have been widely used in systems were aluminium is one of the materials. Among these are:

ñ Chromates and dichromates are very efficient, but poisonous and may oxidise chemicals like glycol.

ñ Silicates are efficient and not poisonous. Silicates, chromates and dichromates all have an inhibiting effect on corrosion of several metals.

ñ Nitrates, molybdates and benzoates are all inhibitors which are suited for aluminium and some other metals.

In systems with different kinds of materials, an inhibitor that prevents corrosion on some of the other materials, will also prevent deposition corrosion on aluminium. Some of these are:

ñ Nitrites which inhibit corrosion on steel and may accelerate corrosion in low concentrations on aluminium.

ñ Borates which is a inhibitor on steel and has in addition a buffering effect, i.e. stabilises pH.

ñ Mercaptobenzothiazole and benzotriazole which are inhibitors of corrosion on copper.

When selecting an inhibitor for a special system, is it very important that all relevant parameters with regards to materials and environment is brought to the inhibitor supplier.