Fellow Shadow Watchers, Following my recent query about etching stainless steel I had the following comprehensive response from Joseph Bellina via the Rete Mailing List which I'm pretty certain reveals all. I'm sure he won't mind me sharing it with the SML.
Many thanks for the responses received, some of which echo Joseph's definitive explanation. "The lyffe so short the crafte so long to learne." - for me certainly! Tony Moss *************************** >Subject: Re: Positively negative. >Sent: 8/4/20 3:23 PM >Received: 8/4/03 7:30 PM >From: Joseph Bellina, [EMAIL PROTECTED] >Reply-To: Rete Mailing List Postings, [EMAIL PROTECTED] >To: Rete, [EMAIL PROTECTED] > >I make an educated guess. I say educated because for most of my life I >have studied the physics and chemistry of surfaces. For a while I was >working on the corrosion of stainless steels and nickel based >alloys...variants on NiCr. > >Stainless does not corrode because it forms a protective oxide of >chromium, one of the components of the alloy. This oxide is very >protective unless the environment becomes very acidic. The solution you >are using is not, but under the edge of the masking material there is a >region that the liquid can reach, but does not allow much circulation >with the rest of the solution. In this crevice, attack of the oxide by >chloride ions tends to increase the hydrogen ion concentration in the >enclosed solution, hence making the solution more acidic. In a sense >the attack is autocatalysed. This kind of crevice corrosion is a >problem around bolts and welds in these alloys. > >You can see a similar type of corrosion if you look carefully at how >rust develops on your car. You will see that the corrosion progresses >under the paint. > -
