Corrosion and Health

This month I am going to stray into Corrosion and Health. This is an area in which, I myself have never done any direct lab experiments: nonetheless, I have been the subject of "experiments" (what I mean by this will hopefully become clear later on!). Actually if you put those two words into a journal's search engine you will get some papers that discuss "health" of chemical plant or railway or road bridges which is not what I want to talk about. It is the health of human beings that I want to cover.

There have occasionally been quite major disasters where corrosion has caused a component to fail or break and the result is injury or even in unusual cases death to human beings. Also sometimes the measures taken to prevent corrosion can cause problems.

As an example, we are aware of lead, chromate.VOCs, etc. in paints and pretreatments causing damage directly or indirectly to human beings, e.g. by altering the environment. But there are other less spectacular examples where direct corrosion could be "dangerous". The three aspects 1 want to cover are:

i) the effect of small levels of metal ions in contact with skin

ii) imbibation of metal ions and

iii) the use of prosthetic devices in the body.

The first is covered in the technical article in this months CM where an investigation has been conducted on nickel release from EuroCoins. From this work an interesting conclusion has been reached. Although under static conditions there were significant levels of nickel dissolved from the coins investigated (enough to cause allergic reactions in sensitive people), as soon as a rubbing is introduced (as might well occur in practice) the level drops to much less than that required.

If I take the example now of jewellery or other decorative or useful metal items that are in constant or nearly constant contact with the skin, pure noble metals generally don't cause problems e.g. gold, silver, (if anything beneficial - see later), platinum etc. However, if you take alloys, it can be a different story; for example palladium is a constituent of some jewellery and some allergic reactions have been seen to palladium ions. In my own case, in the past I have had some skin reaction with metal watch cases and straps but I am pleased to say that the current case (I now use only leather straps) is not causing any problems; there is actually a special grade of stainless steel (316LVM) for use in contact with the human body. I have had difficulty recently where my reading glasses come into contact with my nose and ears - this is probably copper-nickel so relates to the EuroCoins technical article. Regarding imbibation, i historically there have also been problems : with high levels of plumbous ions from i corrosion in plumbing systems causing lead i poisoning. That is much less likely these days.

Aluminium used to be considered dangerous even Ulick Evans stated that it was) but this has been largely discredited (the problems associated with using corroded aluminium saucepans were attributed to bacteria in the pits). We may imbibe copper at a level which are a little higher than ideal (e.g. blue water etc) but occasional high doses do not seem to be dangerous. Silver appears to be effective in destroying bacteria (in Dragon's Den recently a device incorporating "silver nano technology" was "sold" to the Dragons as a disinfecting device!).

Overall then it would appear that the level of problems that are caused by metals in contact with the skin or swallowed is thankfully fairly minimal these days. So that leaves the area of metal components deliberately introduced into the body. I am the owner of a fairly rotten set of teeth - most of those that are left are heavily filled. Hence I have a large amount of mercury amalgam in my mouth. There must be a gradual corrosion of this and I really just trust that it is not doing me any damage. However there is increasing replacement these days of bits and pieces like knees and hips with metal components, sometimes to quite young people, so like my teeth they could be present in the body for upwards of 50 years. Often these contain innately reactive metals such as iron (in stainless steels e.g. 316L)), and titanium alloys e.g. Ti 6AL 4V. These alloys rely (like the back of my watch case) on a good quality, adherent, impervious, defect free, self healing oxide film. Sometimes the film does not always manage to maintain all these requirements and the result is corrosion.

However, although there is obviously going to be a problem with the component continuing to fulfill its function under these circumstances, the released metal ions eg. titanium, nickel, chromium etc are probably produced at a rate and at a level where they are not directly toxic and they disappear out of the system relatively quickly and are not cumulative like mercury and lead.

Well that's probably enough on that topic! Returning to the copper article two issues back there was question asked regarding corrosion of copper pipes laid in concrete. My own experience at the BNF (backed up by recent consultation with Marcel Pourbaix' book) is that this was probably OK unless ammonia was present. Although the pH is somewhat alkaline (11-12) it was still within the passive range of the copper. However my correspondent begged to differ. Views on this or anything else raised in this article are as always welcome: Douglas@Harrbridge.freeserve.co.uk