Vapour corrosion plastics

Table 18.18 summarises the reported vapour corrosion effects of a variety of plastics references to the source of the information is given at appropriate places. 

Table 18.18 Summary of vapour corrosion effects of plastics...

Little comprehensive work has been carried out on contact corrosion, but some results on a range of polymers have been reported by Czech workers". In general, plastics that give rise to vapour corrosion (Table 18.18) will also cause contact corrosion. Some qualification is needed to this statement, however, as much depends on the type of contact and the other ingredients in the polymer, e.g. a paint may give good protection to the metal to which it is applied, but the vapour may cause corrosion of adjacent metal items within an enclosed space. 

Vapour corrosion The best way to pack articles made of metals susceptible to vapour corrosion is in boxes made of metal or of those plastics which do not themselves emit corrosive vapours. If wood cannot be avoided, then the less corrosive kinds should be chosen. Dryness, good ventilation and the inclusion of water-vapour barriers should be sought. Other obvious measures are the avoidance of susceptible metals and the use of protective treatments and paints. 

There are many temporary protectives on the market and it would be impracticable to describe them individually. However, they may be classified according to the type of film formed, i.e. soft film, hard film and oil film the soft film may be further sub-divided into solvent-deposited thin film, hot-dip thick film, smearing and slushing types. All these types are removable with common petroleum solvents. There are also strippable types based on plastics (deposited by hot dipping or from solvents) or rubber latex (deposited from emulsions) these do not adhere to the metal surfaces and are removed by peeling. In addition there are volatile corrosion inhibitors (V.C.I.) consisting of substances, the vapour from which inhibits corrosion of ferrous metals. 

Organic materials Corrosive vapours are sometimes emitted by organic materials used either in packaging or in the manufactured article, and may be troublesome in confined spaces. Some woods, particularly unseasoned oak and sweet chestnut, produce acetic acid (see Section 18.10), and certain polymers used in paints, adhesives and plastics may liberate such corrosive vapours as formic acid and hydrogen sulphide . It may be necessary to carry out exposure trials, particularly where materials capable of liberating formaldehyde or formic acid are involved. Most corrosion problems of this kind can be prevented by using desiccants, and in many cases they are confined to imperfectly cured materials. For an excellent review see Reference 9. 

Even without heating it is likely that in the course of each pass there will be some release of volatile material—like water vapour, plasticizer, extender, or processing aids (anti-static agent or lubricant). There must be opportunity for such substances to be vented and carried away the die or tool must be capable of resisting attack by volatiles (suitably plated if they are corrosive), and designed to prevent an accumulation of released materials. Special hardened... 

Any gas or vapour produced by burning is carefully smelled and recorded, bearing in mind that it may be toxic (polytetrafluoroethylene) or corrosive (cellulose acetate). The pH of any vapour generated is determined by holding Universal Indicator paper or pH indicator strips moistened with distilled water in its path. Plastics producing acidic vapours include cellulose nitrate and acetate, polyester, polyurethane, PVC. Those forming neutral vapours are polyethylene, polystyrene, acrylics, polycarbonate, silicones and epoxies. Alkaline products originate mainly from nylon and formaldehyde plastics. 

Glass reinforced plastics normally have good corrosion resistance against a variety of chemicals and aggessive environments. This is mainly associated with the inertness of the resin, which can be formulated to confer chemical resistance. The main effects of exposure to fluids or vapour is swelling and debonding as a result of absorption by the matrix. Under stress the resistance of resins and fibres to an environment may be reduced through the interaction of the stress with the internal forces. Indeed stress can accelerate the permeation of fluids into the polymer structure in a similar way to temperature. 

Material to be dried per tray. (Capacity of each tray as dry material in kilogram.) MOC of trays and internals. In case of evolution of acidic vapours, the inside should have acid-resistant coating of fibre reinforced plastic or special anti corrosive paint. 

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