Benzoate anions adsorb onto the steel surface and support formation of a passive oxide film, which stops the anodic dissolution that drives rusting. It is an anodic inhibitor, meaning it works by shutting down the metal dissolution half of the corrosion cell, and it is effective at low concentration in a closed water system.
2
Why is sodium benzoate not suitable for mixed-metal systems on its own?
Because it protects ferrous metals well but is far less effective on aluminium and can accelerate attack on soldered joints. A cooling or hydraulic system containing steel, aluminium, copper and solder therefore needs a blended inhibitor package with components targeted at each metal, rather than benzoate alone.
3
What is a vapour phase corrosion inhibitor?
A compound that volatilises slowly from a treated paper or film and deposits on nearby metal, protecting it without direct contact. Steel components packed with VPI paper are protected through storage and transit and need no oil coating to be removed before use. Benzoates are among the compounds used for this.
4
Is it the same material used as a food preservative?
Chemically yes, though food grade is specified to pharmacopoeial or food-chemical limits on purity, heavy metals and chlorinated compounds that industrial grade does not meet. The grade must be stated at enquiry. In industrial corrosion inhibition the antimicrobial activity is incidental; it is the benzoate's surface chemistry that matters.
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