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Chromic Acid Anodize Analysis Help
Forgive me in advance but this will be long.
We recently got approved to start doing chromic acid anodizing here at our facility. The tank was finally set up and I started the solution make up. The operating level is 400 gallons (392 to be exact). I calculated 147 lbs of chromic acid flake to add for make up to give me an initial concentration of 45 g/L. This was done on a late Sunday and I could not analyze until the next day.
The next day, we had a gentleman show up to teach us how to do the scrubber wash down to rinse the filters and have the water go back into the tank. Well there was a malfunction with the solenoid valve and the washdown ended up filling the tank to 1/2" from the top and almost flooding the shop! I pumped out the solution back down to operating level and did the analysis to see how much more I was going to need to add. Even with the dilution (up to 465 gallons), providing my make up was correct, I expected the concentration to be about 38 g/L.
I am using the analysis from Mr. Probert's book "Aluminum How To." My initial pH before the titration was 1.4. At my expected concentration, it should have been around 0.4. I proceeded with the titration and my calculated result was around 4.2 g/L in my tank. I tried again and got the same result. Then I sent a sample out to a local analytical lab and they did an ICP test on it and got 1750 ppm.
I tried two other methods that I found, a spectrophotometric method for Cr(VI) for my Hach spectrometer that gave me 23.1 g/L and the titration from the metal finishing handbook that gave me 44.2 g/L.
In summary: I tried 4 different analysis methods and got 4 different results. I double checked all my math and it was correct. My pH meter is calibrated with a pH 1.68 buffer. I checked the pH of a 45 g/L sample I made myself in the lab and got pH 1.4 as well. What am I not thinking of?? I ran coating weight and salt spray test panels and they looked a nice dark iridescent grey. I got coating weights around 320 mg/sq.ft and my salt spray panels are looking great. I have NO clue what to do next. Please help!
Chemist - Oxnard, California, USA
August 18, 2011
The procedure on pages 117-118 of Aluminum How-To answers in percent by weight. Multiply that by 1.333 and you have av oz/gal. Divide that by 7.5 and you have grams per liter.
I have used this procedure 129 times, biweekly, at a regular client, NadCap approved, and controlled their solution for that length of time with no problem. Regarding pH, you cannot get that low with a common off the shelf electrode, you need a double junction probe.
Stopping the titration at exactly 3.2 requires some experience - when you get close to 3.0, then go three drops at a time, then down to one drop until it hits 3.2 and holds. Kathy at AnaChem in ElSegundo knows how to do this analysis.
Robert H Probert
Robert H Probert Technical Services
Garner, North Carolina
First of two simultaneous responses -- August 19, 2011
Since this is a new bath with no aluminum or trivalent chromium in it, the specific gravity of the solution should give you a fairly accurate estimate of the concentration. If you don't have a hydrometer, you can fill a volumetric flask or graduated cylinder to a known volume and weigh it to determine the SG. You should be able to find a specific gravity vs. concentration chart on line.
Lyle Kirmanconsultant - Cleveland Heights, Ohio
Second of two simultaneous responses -- August 22, 2011
August 22, 2011
Thank you Mr. Probert.
Those are the kinds of silly mistakes that chemists make when they are rushing!! Forgive me for my oversight. I was using the chart on page 116 that was in g/L for my reference and I forgot to look for the units in the analysis sample. I do indeed have a double junction electrode and I just did the analysis again. My pH started at 0.88 and I hit the endpoints at 3.19 and 4.82 respectively. The values were 15.1 and 15.4 mL.
The calculation should then be 15.1 x 0.4 (%wt/vol) x 1.333 (oz/gal) x 7.489 (g/L) and I got 60.3 g/L.
This corresponds very closely to my specific gravity value which was 1.044. I believe in your first response you told me to divide the value I got in oz/gal by 7.5 but I think you are supposed to multiply by 7.5 to get g/L. Am I right? Thanks!
- Oxnard, California, USA
I was wrong, you are right, the analysis seems good. Now let's explore two reasons for failing salt spray. (1) If your anode to cathode ratio is too low, you have converted to too much tri-valent, follow the book, electrolyse to bring down, then do more panels. The cathode area should be about 1/3 to 1/5 that of the anode area.(2) If you are dragging in sulphate from the deox or from a sulfuric acid anodizing tank, be aware the if the chromic acid anodizing solution gets above 1/2 gram per liter of sulphate - you fail salt spray. Maybe number (3) - look at the raw panels with a 10X loop and see pits before you begin. Maybe (4) is the air agitation hitting the parts and displacing solution/current?
Robert H Probert
Robert H Probert Technical Services
Garner, North Carolina
August 23, 2011
We have recently taken on Chromic anodize. I am stuck between methodology. What is the best titration to test for Free chromic acid and total chromic acid? And should I test the dilute chromic seal solution (which calls for testing only total chromic acid) and the chromic anodize solution (needs free and total chromic acid tested) differently?
Orlena Puckett- Tulsa, Oklahoma, USA
November 22, 2011
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