Review of Jerry Harwood's lecture to N&DMS: Temporary Corrosion Protection for Metal Products and Components
Many of the Society's lectures take the audience to the frontiers of knowledge, but from time to time it does members good to be exposed to practical problems where the science is generally assumed to be well understood, but where solutions can nevertheless be complex. In outlining what's available for providing temporary corrosion protection (TCP) for metal products in transit from one location to another or for periods of storage, Jerry Harwood of Technology Packaging explored the main classes of packaging products and how they are applied.
Rusting of ferrous components is the most common example of metal corrosion. As with the corrosion of other metals, rusting occurs by an electrochemical process and explanations of the underlying chemistry can readily be found in standard chemistry textbooks. The TCP system required for each application will vary with the particular metal and will also depend on the timescale of the transit or storage process as well as on the temperatures and relative humidity to be anticipated. For example, a journey within the UK will differ in many important respects from a sea voyage to Africa and each will require a different solution. In general, TCP products should be both easily applied and easily removed whilst being safe to use and cost-effective. The product range starts with barrier foils made of materials such as paper, cardboard, polythene or aluminium foil, singly and in and combination and continues with liquid corrosion inhibitors (LCIs) and volatile corrosion inhibitors (VCIs). Sometimes a dessicant, such as activated clay, which is easily dried for re-use, or less commonly, silica gel, might be included in a composite barrier film to guard against moisture ingress when a long timescale is expected. The variety of TCP systems that can be created is therefore immense and therein lies the skill required to apply the technology to best advantage. For example, barrier films can be manufactured as bags, sheets or rolls of material. Corrugated paper and cartons can be made with or without VCI chips. Sealed enclosures required to incorporate a dessicant can also be created, but the correct amount of dessicant must be carefully calculated beforehand and the supply must be kept fresh before use. Alternatively, VCIs can be captured in bubble film although as with all VCI usage the chemical chosen must be compatible with the metal being protected. For this reason, manufacturers provide compatibility charts to guide TCP system designers.
Of course, TCP systems sometimes go wrong and fast corrective action is required. Cooperation between supplier and customer is essential since poor packaging can often be traced to misunderstandings over the correct way to apply systems. Indeed, Jerry described how on one occasion a temperature difference of 10oC between the packing and storage areas created the potential for corrosion since the dew point within the packaging was reached by virtue of the temperature drop between the two areas. Inclusion of a dessicant solved the problem.
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