Spring Cleaning

Well it's spring and what does one do in spring? The answer is Spring Cleaning! Now one aspect of corrosion is it makes things dirty: tarnished, as well as rusty and cleaning is required in all these cases. This involves interaction with and dissolution of the corrosion product by some chemical but an important aspect is that you want the chemical to minimize its attack on the metal itself.


One example is the cleaning of corroded silver jewellery or precious stones (in the latter case I'm using corrosion in its broader definition of the deleterious reaction of ANY material with the environment). Products to do this are likely to be alkali and/or acidic and include a surfactant and an inhibitor. Some sort of agitation is helpful: in the case of the jewellery cleaning kit in the picture a high frequency vibration (perhaps ultrasonic?) is included. It also assists if there is subsequent mechanical cleaning (note the tooth brush).

Although the average person involved with corrosion may not clean jewellery too often they do, I'm sure, get involved with corrosion product removal in some shape

or form. In the lab this needs to be done on test specimens after they have been exposed eg to measure weight loss. This was an activity that I had to do quite frequently back in my days working for the Nuclear Power Company and was effected in 30% HCl to which was added some black gunk, probably containing tin chloride and Sb2O3. Not too strong acid environments are more acceptable environmentally and citric acid to which EDTA was added is commonly used for copper corrosion products (sulphamic acid is another acid which is effective). Citric acid also has the advantage that it can be used in mixed metal systems e.g. car radiators.

A particular challenge when I was working for the BNF was cleaning up some heavily encrusted silver "Pieces of Eight". My memory of how this was done was a combination of an electrical method (making the work the cathode) and an alkaline solution. Aluminium is a difficult metal to clean because of the oxide's inertness and the extremely high reactivity of the metal underneath if it is removed. The picture of a cistern shows what can happen to aluminium!

The challenge involved in removing that sort of thing with chemicals that are environmentally friendly is really still to be overcome (hydrofluoric acid is effective but not suitable for use outside a lab or factory -any strong acids are outlawed for domestic use an^i even things like EDTA are becoming less acceptable). If you try say caustic soda you may land up with almost no metal left! Phosphonate is useful to clean steel and has the added benefit of removing limescale particularly if a suitable surfactant is added. Once one has cleaned the item there is the possibility of actually reducing subsequent corrosion using a chemical treatment (in the old days using chromate but not now). In the case of the jewellery items such a chemical might be incorporated in the polishing cloths that they recommend. This used to be called passivation (I am not including conversion coatings like phosphate here).


No doubt a lot of chemicals are used up in these processes and it is a big industry whether on the commercial scale or on the domestic side. Maybe there is more scope for refurbishing worn out items rather than throwing them away. Going back a column or two, I was talking about guitar strings and the way that they get tarnished and this dulls their tone What would happen if we tried taking them off, cleaning them and then restringing them? I intend to try this! Anyway I must get back to doing some Spring Cleaning!

Addenda : Thanks are due to Philip Munn and Peter Boden for discussions about this. However the author takes responsibility for any errors!