• Topic #58588 "Salt spray test: Can't get adequate solution flow"

A. Hi Alejandro
I'm not sure if any solution will work better than hot water.
Good luck,

A. You can most likely use a very dilute solution of hydrochloric and DI, I use it all the time to clean films on probes. Hot DI is your best bet for a non-solution based method.

A. Hi &Ounl;zgenur,
While waiting for a specific response, please make sure to read Davey Shaw's comment on collection.
Luck & Regards,


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

A. Are you salt spray testing real parts or test panels, Micheline? If it's test panels, I would suspect damage to them, or non-equivalence of your new source, before I'd suspect water quality.

If it's not the test panels, I may sound like a broken record on this, but some people believe that only deionized water should be used in plating shops anymore. If the water usage rate is not high, a simple 2-bed DI system will solve the problem this time, and forever as well :-)

A. I'm not familiar with any ASTM B117 requirement to run a condensate analysis, but would not necessarily expect the condensate to contain the exact concentration of the bulk solution being aspirated into the (95 °F) tank. You may want to compare your 2% result with the concentration of solution taken from the bottom of the tank and from the "moat" that provides the air-tight seal around the lid.

TRH

A. I politely but strongly disagree with Thomas. The analysis only counts on the collected condensate that does NOT drip off of the parts.
If anything, the salt concentration would be expected to be higher in the condensate since the aspiration of the solution would evaporate it.
How are you checking (measuring) the salt concentration?

A. Hi

We operate SS chambers per the ASTM and it does indeed require that the collected solution be analyzed for both pH and concentration. You can measure concentration by using specific gravity or by using a salt refractometer. The solution must not have touched any parts and it is not to be taken from the water seal around the lid. There are strict requirements in the ASTM as to how the atomized solution is to be collected.

Thanks,

A. This question intrigued me, so I contacted one of the world's experts on this subject (at a UK salt spray cabinet manufacturer) and this was their reply.

This seems to be a very good explanation as it points to several causes that could contribute to the problem and that are worthy of investigation.

Terry

A. A pH of 6.2 is not that acid.

I'd suggest using a freshly calibrated pH probe, and 0.01 N NaOH to adjust the pH to 7.0, if you think that will help. (I don't think it will make much difference, but it won't hurt.)

Insofar as documentation: buy the 0.01 N NaOH from Fisher Scientific, and write the lot number down in a book ... it's hard for me to see how even the toughest auditor could object.

A. Steve,

This sounds very similar to a problem I had a few years back. I used analytical grade 0.1N NaOH ⇦ this on eBay or Amazon [affil links] and HCl for adjustment. Rather than trying to add a fixed amount and see what happened I would take a 100 ml sample and adjust in the Lab and then multiply up to the reservoir size for the addition. In my particular case it was normally found to require about 50-100 ml of 0.1N NaOH in 120 liters of solution. Once adjusted I would retest the pH of the reservoir and ensure it was between the 6.0-7.0 ^ ^6.5-7.2 required by the specification.

By the way, this was related to premature failure of chromate conversion coated samples of 2024 Aluminum.

A. Steve,

Isn't the pH on fallout required to be 6.5 to 7.2?

And yes, I would think that a low pH will affect your results.

We use 0.1N reagent grade NaOH or acid to adjust. Make small additions

A. You must maintain your collection pH between 6.5 - 7.2 if you wish to perform testing per ASTM B117 (and also avoid false failures and audit findings). Is the sample exposed to open air for any period of time prior to measuring the pH? If so, it may be possible that the solution is absorbing carbon dioxide from air and the pH is decreasing accordingly.

Another question: does your pH meter automatically compensate for temperature differences? If the meter is assuming 77 °F, but your sample is 95 °F, there will be enough error to make you believe you are out of tolerance.

As mentioned before, increase your salt solution with 0.1N NaOH prior to introduction into your chamber. You want to target the higher end of the tolerance since pH of the collected fog will almost always be lower than your salt solution. Also measure with a fresh calibration and a suitable pH electrode/meter. We prefer to calibrate prior to each use.

A. That is because of the atomization of the solution: some of the moisture and only a bit of the salt goes out the vent.
I always made up my salt solution on the low side so that the condensed salt solution was on spec.
Carbon dioxide caused me a bit of pain in controlling the pH of the collected solution. It required adjustment a couple times a week.

A. Hi Jem. We appended your inquiry to an earlier thread which should offer you some insights. See the subheading "Our chamber rusts the test parts in too few hours" above. Good luck.

Regards,

A. Hi Jem,

You really need to tell us what you are testing, both base materials and coatings. At the moment it sounds like you are testing on steel, possibly zinc or zinc alloy plated steel?

Anyway the failure to monitor the pH means that you cannot claim to be testing in accordance with ASTM B117. The specification requires that you keep a daily log of the pH from the collectors. So regardless of the effects of pH you need to monitor it anyway.

A. Hi Jem.

1. The corrosion behavior of different platings on different substrates in acidic or alkaline environment are different. Therefore, it is imperative that you maintain pH at 6.5 ~ 7.2 (ASTM B117) to consistently achieve and interpret salt spray results.

2. Interpretation of pass/failure of SST will depend on the standard that the plated part has to adhere to. There is no one reference or standard that covers all. As an example, the mention of white rust and red rust suggests zinc plating on steel substrate. If you are plating to ASTM B633 ( Standard specification for electrodeposited coatings of zinc on Iron or steel ), it states:

Corrosion Resistance Requirement
    Types        Test Period, hours

     II                 96

     III                12

Thank you for the advice. I am now maintaining the pH Level of the collected solutions and already identified the best pH reading for the Salt Solution in order to have a reading of pH 6.5-7.2 for the collected solution.