An unusual question arose recently about how to clean old and used electronic components. The particular component in question was an old socket (base) for a radio valve (tube), which had been put in to an ultrasonic cleaner in water containing a cleaning agent (specifically sold for ultrasonic cleaning of the more reactive metals like aluminium and copper). Very quickly after the cleaning started it was noted that metal skirt on the valve base was turning black and the cleaning was stopped.
The speed at which this effect took place surprised everyone and we don’t have a good understanding of what went on. What we do believe is that the problem may have been caused by some sort of galvanic corrosion process caused by having a number of dissimilar metals in an aqueous solution. The base had a metal skirt (which was plated), the valve pins (also plated) and solder (tin/lead?) on the bottom of the pins.
Whether the effect could have been avoided by using a different cleaning additive, or a lower concentration of additive or a lower temperature are all valid questions. It may well be that using an organic cleaning agent would have been a better approach, but since the demise of the Freons and chloro-carbon solvent cleaning agents there has been a strong trend to go to aqueous solutions and many of the commercial products are extremely good for general use. The Freons and chloro-carbons solvents were good cleaning agents and their high density suited their use in ultrasonic cleaners. Of the organic solvents which are still available, pretty much all of them give rise to Health and Safety considerations and environmental concerns and some have adverse effects on polymeric materials.
The conclusion we have come on this one is that when cleaning multi-metal parts which have critical surfaces (including electro-mechanical components with solder on them) it may be best to avoid aqueous solutions unless you are prepared to go through a proving process first. The cost-effectiveness of re-using such components may be an issue unless it’s a case of working with obsolete or other hard to find parts.