1897 one penny high tide variation

[Diaconis]

On 12/2/2016 at 10:59 AM, 1949threepence said:

No, not at all. I agree with you 

Not sure about the scientifics of pitting. I'd imagine it is a specific response to a given environmental factor - possibly water. Once that factor is removed via placing the coin in a warm, dry setting, there would be no further spread.  But not sure. Maybe somebody else can fill us in on this one?    

Mike is probably spot on here, (no pun intended) humidity and acids are the cause.

A chap in China made a study of the effects of sweat causing pitting corrosion on brass coins using synthetic sweat. Here's a snippet which might be of interest.

"Corrosion behavior of brass coinage was investigated in synthetic sweat solution by electrochemical measurement and surface analysis methods including scanning electron microscope (SEM) and energy dispersive X-ray spectrometer (EDX). It is indicated that chloride ions in sweat solution accelerate the anodic active dissolution of brass, which is the main reason for pitting corrosion and dezincification corrosion. Meanwhile, lactic acid and ammonia water also promote the anode reaction. The corrosion products on the surface are mainly composed of basic copper chloride, cuprous oxide, the complex consisting of urea in association with copper, and few lactate ions.  The kinetics of pitting corrosion development obeys the following equation of J0=0.3735(t+185.93)−1/2, and the process is controlled by the dissolution of salt deposited on pit surface."

So, it would seem that if your coin is dehydrated then, the brake is on for pitting. 'Handle with gloves' and keep them in a very dry environment would seem the way to go here.

[blakeyboy]

This is all very similar to what goes on in primary cells.

Or indeed when a galvanised roof has a bad scratch- once you have the two different metals exposed to rainwater, a cell is formed

that is basically short-circuited, so a toroidal current path forms, the chlorides get to work,  and the roof has a hole in it in a shorter time than if

it hadn't been galvanised.....