Thanks I always learn something new from this site.

Tom
www.kegkits.com

-----Original Message-----
From: Vanagon Mailing List [mailto:vanagon@GERRY.VANAGON.COM] On Behalf Of
David Beierl
Sent: Thursday, April 22, 2010 12:44 PM
To: vanagon@GERRY.VANAGON.COM
Subject: Re: Hose clamps..

At 03:19 AM 4/22/2010, Tom Hargrave wrote:
>Stainless is made corrosion resistant by a thin oxidation layer on the
>surface of the metal called passivation. Spot welding will destroy the
>oxide layer protecting the SS from rust but it takes very little time,
>literally seconds, for the weld area to re-passivate and protect the
>welded area from corrosion.

It's a little more complicated than that.  Stainless steel is subject
(variously in different grades and different environmental
conditions) to pitting corrosion, crevice corrosion, contact corrosion,
fretting corrosion, stress corrosion cracking and intergranular corrosion.*
Seawater or mixed seawater/air exposure is one of the more severe
environments for many forms of s/s
corrosion.    In the case of welding, intergranular corrosion is the
culprit.  Quoting
<http://corrosion-doctors.org/Forms-intergranular/intergranular.htm>:

>A classic example [of intergranular corrosion] is the sensitization of
>stainless steels or weld decay. Chromium-rich grain boundary
>precipitates lead to a local depletion of Cr immediately adjacent to
>these precipitates, leaving these areas vulnerable to corrosive attack
>in certain electrolytes.**  Reheating a welded component during
>multi-pass welding is a common cause of this problem. In austenitic
>stainless steels, titanium or niobium can react with carbon to form
>carbides in the heat affected zone (HAZ) causing a specific type of
>intergranular corrosion known as knife-line attack.
>These carbides build up next to the weld bead where they cannot diffuse
>due to rapid cooling of the weld metal. The problem of knife-line
>attack can be corrected by reheating the welded metal to allow
>diffusion to occur.

* Some of these (fretting, pitting, crevice, contact) can be astonishingly
rapid -- I have personally observed a s/s cooking pot about 1 mm thick go
from apparently perfect to perforated (~2 mm hole, bigger on the inside) in
a few days.  Apart from fretting which is simply the continual removal of
the passivating layer, I personally think that oxygen depletion in the
corrosion zone is an important factor in these corrosion types

** Seawater or mixed seawater/air exposure is one of the more severe
environments for many forms of s/s corrosion.

Yours,
David