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Heat Tint Colour Related to Chromium Depletion in Duplex
5

Heat Tint Colour Related to Chromium Depletion in Duplex

Heat Tint Colour Related to Chromium Depletion in Duplex

(OP)
The application is a desalination plant.

Materials are Duplex SAF 2205 and Super Duplex SAF 2507.

Question : Is the colour of the heat tint able to be used to quantify the extent of chromium depletion in the weld? Further is the degree of chromium depletion quantifiable such that the design life in sea water can be quantified?

What I am driving at is if SAF 2205 heat tint is light straw in colour is this OK in moving sea water? Whereas dark straw may not be and thus should be rejected?

Is there a standard that covers the colour of the heat tint and what should be accepted and what should be accepted for 2205 or 2507?

Geoffrey D Stone FIMechE C.Eng;FIEAust CP Eng
www.waterhammer.bigblog.com.au

RE: Heat Tint Colour Related to Chromium Depletion in Duplex

2
The thinnest, yellow heat tint may be amorphous and considered protective.
 
For water service, straw-yellow heat tint on austenitic & duplex SS welds is often acceptable, "up to and including AWS D18.2 No. 3 on the colour reference chart is often considered acceptable." [color chart shown also]
http://www.nickelinstitute.org/multimedia/technical_support/technical_literature_pdfs/11000_reference_book_series/11026.pdf

"Abstract: Corrosion resistance and artistic appearance of passive chromium oxide coatings developed over stainless steel by chemical conversion methods are revisited. Preparation technique and thermal processing of the films were optimized to improve protection efficiency and the control of colour development - from yellow to green through violet...Enhanced passivation behaviour of thermally processed films is related to the formation of a spinel-type phase after migration of manganese from the bulk steel to oxide coating surface." from
'Corrosion resistant coloured chromium oxide coatings on stainless steel' by M.O. Figueiredo & M.J. Carmezim, Pigment & Resin Technology, Volume 27, No. 4 Pages 243-246 (Aug 1998). [full article costs money]
http://www.emeraldinsight.com/Insight/viewContentItem.do?contentType=Article&contentId=876837&hdAction=lnkhtml

"Air passivation of stainless steel
May , 1998
...The surface of stainless steel can be passivated by heating in air. However, the temperature and dew point are very important. A smooth oxide film is formed on 316L stainless steel at 450oC and a dew point of < 0 oC..." -- Don Mattox http://www.finishing.com/faqs/sstpass.html
I took a thin films & vacuum coatings course from him a few years back. He has very strong credentials:
http://www.svc.org/TC/TC07/CFP0750Anniversary.html

RE: Heat Tint Colour Related to Chromium Depletion in Duplex

2
I have yet to see anyone replicate the color tints on duplex, but for the light (thin and acceptable) cases I would expect them to be the same as for 316L.

There is a much more detailed set of colors, but I can't find it now.  Both the ASME BPE and SEMI have done work to show the effect of heat tints that are lighter than AWS #3 in much finer steps.

Removal of the heat tint by pickling and then passivating the system will always yield superior corrosion resistance.
My personal stance is that if the tint is light gold to yellow and transparent then the negative effects are minimal.  I rely on being able to see through the film as an indication that it is light enough.

= = = = = = = = = = = = = = = = = = = =
Rust never sleeps
Neither should your protection
http://www.trent-tube.com/contact/Tech_Assist.cfm

RE: Heat Tint Colour Related to Chromium Depletion in Duplex

From 'STAINLESS STEEL IN PRACTICE'
http://www.butting.de/english1/seiten/PDF/down-d/WPraxis/Werkstof.pdf

Re heat tint on SS from welding ('blooming'):

"A generally valid statement about leaving
the thin oxide layers in the form of
blooming cannot be made as here the
total system of media of attack, material
and surface condition have to be judged.
The February 1993 edition of the
DIN 50930, section 4, paragraph 6.2
states as follows: Thin straw coloured
oxide films will not cause corrosion in
waters the composition of which corresponds
to that of drinking water and the
temperatures of which do not exceed
100 °C. In pressure bearing systems at
increased temperatures and generally
in systems which, according to sections
5 and 6, are noted for properties that may
increase the possibility of a corrosion
attack, straw coloured blooming may
have a detrimental effect on the corrosion
resistance of the material. In general,
blooming up to a shade of blue may be
left."
-- p. 10

See also 'Chart 3: Influence of surface condition on the pitting corrosion resistance of an austenitic CrNiMoTi steel [Werkstoff 1.4571, a Ti-stabilized 18Cr-11Ni-2Mo Austenitic SS ~ 316Ti]'
-- p. 11

Chart 3 shows that yellow tinted has much better resistance than darker tints & is only slightly poorer than a 120-grit ground surface. Polished is better, and pickled is even better.

So, if using duplex SS in conditions for which 316 or even 304 would be adequate, the yellow tint may not matter. For more rigorous conditions, pickle + passivate.

RE: Heat Tint Colour Related to Chromium Depletion in Duplex

I guess that I will make another comment.
You cannot mechanically remove the discoloration from stainless welds by simple abrasive methods.
A weld 'cleaned' with a wire brush or abrasive flap wheel will often have worse corrosion resistance than the original.  This is because all you have done is to imbed the oxide and chrome depleated metal into the surface.

Cleaner is always better.

= = = = = = = = = = = = = = = = = = = =
Rust never sleeps
Neither should your protection
http://www.trent-tube.com/contact/Tech_Assist.cfm

RE: Heat Tint Colour Related to Chromium Depletion in Duplex

Shell have a heat tint reference document for duplex stainless steel calibrated against corrosion testing.  Having established a heat tint reference, the next question is how are you going to inspect for it during piping fabrication, say?  Pickle it and have done with heat tint acceptance criteria.

Steve Jones
Materials & Corrosion Engineer
http://www.pdo.co.om/pdo/

RE: Heat Tint Colour Related to Chromium Depletion in Duplex

If you are going to rely on heat tint criteria you need two things:
1. Oxygen sensor on you purge gas vent, recorded for every weld.
2. A 20' borescope so that every weld can be visually inspected.

If you aren't going to look at them there is no reason for having a criteria.

= = = = = = = = = = = = = = = = = = = =
Rust never sleeps
Neither should your protection
http://www.trent-tube.com/contact/Tech_Assist.cfm

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