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Salt Spray Test and Scribe Marks

Current question:

June 9, 2021 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. A batch of galvanized steel coupon samples went through cyclic salt spray corrosion tests. Scribe marks were made on the surface before these samples are put inside the cyclic salt spray unit.

It was observed that red rusts start to appear after 1000 hrs or so on some samples, but oddly, the scribe marks show no red rusts. It is just as if the scribe marks are not there.

I am trying to find out why. The scribing removes the galvanization and exposes the steel underneath. I understand the zinc nearby will provide some protection to the scribed areas due to the self-healing nature of the galvanization. But I was expecting the scribed area to show red rusts first regardless. I am pretty sure the scribing is deep enough. Any suggestions?

Peter Zhang
- Baltimore, Maryland


June 2021

A. Hi Peter. While awaiting replies you might find Glen Larson's comment interesting.

Luck & Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
Aloha -- an idea worth spreading
finishing.com - Pine Beach, New Jersey


June 11, 2021

A. 1000 hours is a truly exceptional number of salt spray hours for conventional hot dip galvanizing without a topcoat. Are you sure that you scribed through the various intermetallic layers - eta, zeta, delta, and gamma as set forth by the American Galvanizers Association (galvanizeit.org)? A photomicrograph of the scribe would tell you. In this great website and at the AGA website you will find all the information you could want on the sacrificial nature of zinc and what to expect in both accelerated and real world testing.

tom_rochester
Tom Rochester
Plating Systems & Technologies, Inc.  
supporting advertiser
Jackson, Michigan, USA
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Closely related Q&A's, oldest first:

2000

Q. When salt spray testing galvanized powder coated parts can anyone identify the critical factors that effect salt spray results. Please have data to back up information.

When scribing parts should the scribe penetrate the galvanized, please advise.

We have seen results where the galvanized coating has been eroded during the salt spray test. Is this normal?

Marc Burger
- Louisville, Kentucky USA


2000

A. Marc,

Hopefully you will get more technical replies from others, but you are experiencing the exact reason why cyclical testing or prohesion testing is favored by those who coat over galvanized material.

Simply stated, the galvanization does not have a chance to "repair itself" while under constant exposure in the salt spray. In real life (field exposure), at some time the coating will dry out, allowing this repair process to occur and therefore improving corrosion resistance with galvanized material.

Glen Larson
- Redmond, Washington


2000

A. Marc

The s/s test requires that you scribe thru the paint film to the zinc surface. You do not cut thru to the steel. You will find that the zinc will chemically erode until red rust forms in the scribe. The performance of the paint will depend upon pretreatment. Suitable yellow chromate will give >1000 hours ASTM B117 [affil. link to spec at Techstreet] salt spray. So will correctly applied zinc phosphate. Cleaner-coaters ("iron phosphate") depend heavily upon the final rinse used for performance but are unlikely to exceed 500 hours. Painting HDG steel is not straightforward as the zinc corrodes readily in salt and hence the paint film will be undercut from scribe or damaged areas, hence pretreat is vital for a durable finish. R

Roger Bridger
- Croydon, UK



To minimize search efforts and to offer multiple viewpoints, we combined previously separate threads onto this page. Please forgive any resultant repetition, failures of chronological order, or what may look like readers disrespecting previous responses -- those other responses may not have been on the page at the time :-)



How does the width of the scratch affect the corrosion results in salt spray

2003

Q. How does the width of the scratch affect the corrosion results of painted galvanized steel in the salt spray test ( ASTM B117 [affil. link to spec at Techstreet] ). Does anybody know of a prescribed width ? What is used commonly ?

Best regards,

Piessens Peter
- Belgium


2003

A. ASTM D1654 [affil. link to spec at Techstreet] and GM9102P describe methods of scribing. ASTM is presently reviewing D1654 for possible revisions to improve the consistency of the scribe. Studies have shown that the depth of the scribe will make a significant difference.

Cynthia L. Meade
- Sylvania, Ohio, USA


2003

Q. Thanks very much Cynthia. Currently we are scribing the galvanised panels with a stanley cutter which gives of course a very thin scribe. Corrosion results are always perfect (painted galfan, Zn 95%/5%Al). However, when making a wider scribe (e.g. 1 mm) heavy blistering occurs around the scribe. What do you usually do for ASTM B117 ?

Best regards,

Piessens Peter
- Belgium


2003

A. Unless otherwise specified by the customer, we use ASTM D1654 which describes the scribe tool as a "straight-shank tungsten carbide tip, lathe cutting tool or carbide-tipped pencil-type".

Cynthia L. Meade
- Sylvania, Ohio, USA


July 3, 2008

A. What an interesting question! I believe the damage dimension must effect the result but this is often not referred to in Standards. I have completed work in my lab which shows the effects on chromate based paints on aluminium and here we have clearly seen an effect. The ISO standards for filiform corrosion (ISO 4623-2 [affil. link to spec at Techstreet] & EN 3665 [affil. link to spec at Techstreet]) specify damage of 1 mm width AND that the paint film at the edges should be smooth. Both factors will affect time to corrosion.

What is your conclusion?

Peter Morgan
- Filton, Bristol, UK



To minimize search efforts and to offer multiple viewpoints, we combined previously separate threads onto this page. Please forgive any resultant repetition, failures of chronological order, or what may look like readers disrespecting previous responses -- those other responses may not have been on the page at the time :-)



Evaluation of Salt Spray test results: correlation of blisters with the scribe

July 17, 2012

Q. The query is regarding the evaluation of Salt Spray results.

Salt Spray resistance Test Method being followed: As per ASTM B117 [affil. link to spec at Techstreet]

Evaluation Methods being followed:
- Blisters: ASTM D714 [affil. link to spec at Techstreet] - 87 - Standard Test Method for Evaluating Degree of Blistering of Paints
- Scribe & Unscribed specimen evaluation: ASTM D1654 [affil. link to spec at Techstreet] -79 A.

Our acceptance level for the Scribed specimen - Mean failure distance from Scribe: 2 mm & rating of 7.

We need to know the acceptance level in terms of Blistering (Size & Frequency) corresponding to the above scribed specimen acceptance level.

Deva Deva
- Gurgaon, Hariyana, India

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