Rudy is correct in that you can repassivate but:
Under anodic passivation in nitric, the SS will continue to etch. THERE WILL
BE NO PASSIVATION!  Under immersion conditions (No rectifier), nitric
passivates by removing free iron from the surface. As the etching occurs,
the surface area of the tank increases resulting in greater difficulty
forming and maintaining the oxide film. This is due to the fact that you get
intergranular corrosion. Chrome concentration at the grain boundaries is
much higher than in the bulk grain. By attacking these areas preferentially,
you leave behind a lower overall chrome content and a weaker film  The
electropolishing reduces the surface roughness to a minimum, making
maintenance of the oxide film much easier. This is a known phenomenon with
anodic passivation and has been experienced.


Best Regards,
Rob Durante
OMG Fidelity
Southern California
(714) 540-7388

-----Original Message-----
From: [log in to unmask] [mailto:[log in to unmask]]
Sent: Friday, February 02, 2001 6:49 PM
To: [log in to unmask]; [log in to unmask]
Subject: Re: [TN] Anodic Protection of Electroless Nickel

In a message dated 2/2/01 3:55:25 PM Pacific Standard Time,
[log in to unmask] writes:

> *       Have there been any incidences in the past where the system has
been
>  left on during nitric passivation? In this case, the tank has been etched
>  and will not form a passive regardless of what is done to it unless it is
>  re-electropolished.

I would repectively disagree with the above.  It is my understanding that
you
can passivate Stainless, as long as there is metal remaining.  Passivation
forms an insoluble oxide coat, mostly of Chromium.  If the surface has been
etched away, it is my understanding that the Nitric eats/etches until it
gets
to a surface that it can passivate, and stops....and does the job.

Rudy Sedlak
RD Chemical Company

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