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topic 30106 p3

Zinc plating thickness vs. salt spray resistance

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A discussion started in 2004 but continuing through 2019

March 11, 2018

Q. In Internal salt spray testing we have found red rust in 144 hours in respect to [required] 264 hours.
Plating type - Zinc plating with yellow passivation.
Plating thickness observed -- 9 micron on same batch part in respect to requirement of 12 micron minimum.
Testing condition - As per ISO 9227 -2012
Part Type - M10 -Screw
Photo & test Reports - Refer below

So can you please share the probable causes of above issue, accordingly I am planning to visit at supplier to validate the same.

Regards,

Ansh Mishra
- Bawal Haryana, India


March 2018

A. Hi Ansh. I think the failed thickness check rather than the failed salt spray test is the heart of this matter!

If the screws are supposed to be plated to 12 micron thickness, but the supplier is not plating them for a long enough time to achieve that requirement, something is very wrong and must be fixed before you can proceed. The vendor perhaps justifies theirself by believing that your spec is incorrect or poorly written ... and they may be right or wrong. But whether they agree to plate to your spec or you agree to change the spec, STEP 1 is that the plating thickness must conform to the spec; if it doesn't, all else is simply nonsense :-)

Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
finishing.com - Pine Beach, New Jersey
Striving to live Aloha


March 14, 2018

A. Hi Ansh. As Ted said, first verify why the bolts have less thickness than specified. It could be that your supplier didn't know the spec (it wouldn't be my first), mistakes in plating time or current, low zinc/low anode surface, low bath conductivity (potassium/ammonia?)...

Not knowing your supplier, you can make many assumptions, but what you have is a part not meeting your spec (12 microns minimum, 264 hour of SST without corrosion of base metal). Go with that and listen to what they say :)

Best of luck!

Daniel Montanes
TEL - N FERRARIS - Canuelas, Buenos Aires, Argentina



February 5, 2019

Q. Hello. Can we achieve 480 Hrs of salt spray life with 8 Micron coating thickness?
If yes, with which process?
Kindly guide.

Pankaj Kumbhare
Rishi Laser Ltd - Pune, Maharashtra, India


February 2019

A. Hi Pankaj. I doubt it. But at the least, 480 hours sounds equivalent to very severe exposure (25 microns) rather than moderate (8 microns) exposure. Can you tell us what spec you intend to evaluate the salt spray life according to, and give us some background info rather than a hypothetical? Thanks.

Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
finishing.com - Pine Beach, New Jersey
Striving to live Aloha


February 18, 2019

Q. Hello Mr Mooney.
Thanks a lot for your reply. Actually we had an inquiry from our customer who is dealing in arial work platforms which are working in environment with moderate salinity & expecting a life cycle of 5 to 15 years (C4). Customer is specifying Zn-Ni plating and 8 Micron plating thickness.
Our supplier is confident of achieving it, I just want to confirm it.

Pankaj Kumbhare
Rishi Laser Ltd - Pune, Maharashtra, India


February 2019
30106ext

A. Hi again Pankaj. Thanks for the additional info. The other people on this thread have been discussing zinc plating; zinc-nickel plating is a different process and is much more corrosion resistant. On topic 28851, Asif Nurie asserts that 500-1000 hours is possible, but some other posters reported failure within that time frame. The way to confirm the ability to comply is to plate sample parts and test them for 480 hours, then evaluate them in accord with some specific specification. Good luck.

Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
finishing.com - Pine Beach, New Jersey
Striving to live Aloha



April 13, 2019

Q. if we deposit 1 micron thickness on parts then How much life will achieve?

Umesh W [last name deleted for privacy by Editor]
QA - Aurangabad, Maharashtra, India


April 2019

A. Probably zero, cousin Umesh. I've never heard of anyone trying to use only 1 micron of zinc plating. 5 microns is specified for "mild" service conditions. No abstract questions please!

Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
finishing.com - Pine Beach, New Jersey
Striving to live Aloha


June 15, 2019

A. Hi Umesh,

I'm not sure what life 1 micron will achieve but your business life expectancy with your customer will be zilch if he finds out you were asking..  :)

Khozem Vahaanwala
Khozem Vahaanwala
Saify Ind
supporting advertiser 
Bengaluru, India
Saify Ind



June 28, 2019

Q. I have a customer that is requiring a Zinc Black .0005" plating. My question is does .0005" provide more corrosion protection than .0002?

Andy Hanks
Fastener Distribution - Erie Pennsylvania


June 2019

A. Hi Andy. Yes! .0002" (5 microns) is for "mild" service per ASTM B633 [link by ed. to spec at TechStreet] and related specs, and .0005" (13 microns) is for "severe" service. (.0003" is for "moderate" service and .0010" is for "very severe" service).

Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
finishing.com - Pine Beach, New Jersey
Striving to live Aloha



June 28, 2019

Q. Hi I am from a metal stamping company in Malaysia.
I have a customer making snowtire chain wear plates using Boron Steel from SABB Boron 22. After stamping it goes thru Oil Quenching + Blue Zinc Plating (did not specified thickness) + Tempering.
Is it possible to achieve "No white rust after 200 hours in 5% NaCl atmosphere pr salt test standards ISO 9227 [link by ed. to spec at TechStreet].
Regards,
Lawrence

Lawrence CHOONG
Y&L Metal components Sdn Bhd - Balakong Malaysia


July 2019

A. Hi Lawrence. If you can plate them, you should be able to get 200 hours with a good chromate conversion coating. But I think you might want "clear" rather than "blue", and I'm not sure you will be able to plate oil quenched parts unless you have a very good cleaning cycle. Further, this application sounds like "very severe" service", such that zinc plating thickness should be specified rather than being satisfied simply with a white rust salt spray test. Good luck.

Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
finishing.com - Pine Beach, New Jersey
Striving to live Aloha



August 1, 2019

Q. We want coated brackets with zinc. Brackets can be welded or bolted, generally L shaped. We require brackets for severe environment conditions. So I want to ask about the thickness of coating and salt spray test durations for white and red rust, we should prefer?

Dhanaj Singh Panwar
- Bhopal, India


August 2019

A. Hi Dhanaj. Per ASTM633 and some related industry standards, "severe" service should have 13 microns, 'very severe' (bold atmospheres, cleaners, saline solutions, denting, scratching and abrasion) 25 microns. It seems to me that automotive and some others tend to use alloy platings like zinc-nickel of the next thicker thickness instead (8 microns of alloy plating for the severe stuff and 5 microns for the moderate).

192 or 200 hours to white rust seems to be pretty typical. As for red rust, I'd concentrate on the thickness rather than the hours to red rust since salt spray testing tells us little to nothing about real-life endurance of zinc coatings because the corrosion mechanism is fundamentally different.

Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
finishing.com - Pine Beach, New Jersey
Striving to live Aloha



August 22, 2019

Q. My question is: is there any relation ship between plating thickness (any type) and salt spray hours or any other factor also contribute for salt spray hours.

sateesh .n
nash - bangalore, karnataka, India


August 2019

A. Hi Sateesh. You are asking an extremely broad abstract question which is probably more appropriate for a book than for a brief public forum response :-)

The short answer is, no, the salt spray resistance is generally not directly proportional to the thickness. In zinc and zinc alloy plating the quality of the chromate conversion coating is probably a bigger factor in salt spray resistance than the thickness of the zinc plating. But zinc is anodic to iron so there is at least some degree of proportionality between plating thickness and salt spray hours because the zinc plating will gradually dissolve.

In other types of plating, where the plating layer is cathodic to the substrate, such as nickel plating on steel, the 'porosity' of the plating is far more important than the thickness because corrosion begins as soon as the salt solution reaches the steel substrate. As a general rule, increasing the thickness of the plating probably decreases the porosity, but a properly applied fairly thin plating layer may be less porous than a poorly applied thicker layer.

Please introduce yourself and your situation; people can probably help you with a specific problem but, much as they might wish to, they can't condense a lifetime of experience into a couple of paragraphs for you.

Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
finishing.com - Pine Beach, New Jersey
Striving to live Aloha



September 25, 2019

Q. Our client want that surface treatment MFZn2-C and SST required 240 Hrs. Is it possible? If yes, what is the procedure?

Jamal Yousuf
- Karachi, Sindh, Pakistan


September 2019

A. Hi Jamal. What company or international standards body are you referring to with that spec? Different automobile companies have specs that may use similar abbreviations and which look sort of the same but which are not the same specs. In some specs the number after MFZn is thickness in microns, but I'm sure that's not the case here; it's perhaps a Honda spec and means exposure class 2.

Under ideal conditions, yes, a well controlled zinc plating process can probably deliver 240 hours, quite possibly more. Does the specification mention 240 hours or is that only mentioned on the drawing? Thanks.

Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
finishing.com - Pine Beach, New Jersey
Striving to live Aloha



WSS-M1P85-B2 zinc plating is failing 120 hour salt spray

October 28, 2019

Q. Hello, Jason here from the automotive sector.
I have a door hinge of stamped steel (EN 10149-2-S420MC) that is treated per WSS-M1P85-B2 (electrolytic zinc 8µm thin film passivate).
I am seeing red rust around 50-60 hours; spec is 120 hours.
The film build using electrometer is above requirement at ~13µm. Documentation from the supplier is good.
I'm traveling to the plater to [investigate] root cause. What should I be looking for?
My plan is to review the spec and process with the plater. Then place parts in the salt spray chamber (marked) and watch them on-site for three days.
Any insight would be appreciated.

Jason Naffziger
Advanced Quality Engineer - Auburn Hills, Michigan (USA)


October 2019

A. Hi Jason. I think you should try to track one batch of parts from the receiving dock to the salt spray cabinet. Very often the flow sheets, no matter how carefully written, don't describe what really happens. "Walking the line" often immediately reveals what is wrong ("No good decision was ever made from a swivel chair" -- General George S. Patton).

Some thoughts: The spec probably calls for no "white rust" until 120 hours, let alone "red rust". The high salt spray hours bragged about by the process vendors always apply to ideal shaped parts and after excellent mechanical preparation. Has the plater made any comments about "design for plating"? Are the hinges properly deburred? Are these "hinges" pre-assembled, such that capillary action may mean that acids are drawn in from early process steps are carried through the process? Is there a particular area on the hinge which rusts first? Has the plater recently changed chromate suppliers?

Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
finishing.com - Pine Beach, New Jersey
Striving to live Aloha



November 14, 2019

Q. We have bung inserts made from EN19 steel and our supplier provides trivalent coating of 9-11 microns thickness. The bung inserts are exposed to hard water and we see rusting on finished components after use as well as when stored at 40% RH. Does the grade of steel make any difference for trivalent coating? If we change steel to IS 20162 (hot rolled structural steel)? Or should we go for Ni-Cr plating? The bung comes in contact with propane gas.

Pradeep Kamat
Composite LPG Cylinders - Mumbai, India

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