Alodine / Alodize Tiger Stripe problems 1

[DennisDiamond]

Hi

Having some Tiger stripe issues with my alodize process. Has anybody out there had these problems? I can't be the first. Various material types, various process times, various rakes and raking methods, Same part same day -> different out come. Seems to more frequent on larger parts with holes (shape? or just more surface area to notice?).

Have completely redone batchs which had some tiger striped and had complete different results per part, good became bad, bad became good, all went bad, all went good. Having alot of trouble, too many people with too many "Maybe it's the..." with myself included.

basic Process
Degrease-> Alkaline clean(turco)-> Rinse-> Deoxidize Amchem(6-16(Nitric Acid)-> Rinse-> Alodine(1200s)-> Rinse-> Dry

test pieces pass, parts don't


Thanks in advance
Dennis

 

[kenvlach]

All other things being the same, I believe that it is due to variations in residual stress.

The chemfilm bath (chromic, nitric & some fluoride activator) preferentially 'bites' into the more stressed regions. This is more often seen in sheet metal (maybe some rippling when reverse rolling or otherwise straightening after uncoiling?) than in billet. Common around punched holes.

Sometimes adjusting the chemfilm pH helps, sometimes mechanical agitation helps, sometimes nothing seems to help. The nice flat test panels always always look good..
Ken

 

[DennisDiamond]

thanks for the ideas, really appreciated


The stripes sometimes appear to 'run' from the holes, Not always thou, so I'm not even sure if the holes are involves or not, most of our parts have holes so it's difficult.

After the parts are routed and drilled most are heat treated in our salt bath. Shouldn't this relieve most of the stresses? However there are still additional formings post heat treat but the drilling tasks are all prior.

If residual stress is a major factor wouldn't reprocessing the parts just result in more tiger striping. We've been reprocessing the parts and they usually come out fine on the first rework (some are redone 2-4 times however).

Again thanks for any comments

 

[kenvlach]

Maybe I didn't fully visualize what you meant by "tiger striping."
The residual stress effect on chemfilm tends to create a "halo" around holes.

Discoloration that runs vertically downward from holes is a sign of improper draining between processing steps. It is usually called "bleed out." Most obvious with anodizing dyed black, with holes of lenghth >> diameter. Residual sulfuric acid anodize solution drains out in the hot dye solution, leaving a tell-tale white streak at the hole. If the holes, need to be anodized (can't be plugged), then either flush them out with a water spray gun, alternate warm and cold rinsing, or simply rinse better. Helps to hold the rack of parts at several angles for better draining between steps, also. The holes can be blown out with water or filtered air.

Note that bleed out during prior steps can affect the surface being chemfilmed, as well as bleed out during chemfilm, as well as bleed out of residual chemfilm solution after final rinsing.

 

[DennisDiamond]

Could be inadequate rinsing, but this is sheet metal (1"x1" to 15'x3') how much fluid could they hold, the holes are < 1/16" thick on average. Parts are rinsed with spray for min of 30sec (checking for water breaks) then submerged for 3-5min Maybe I can post a picture of it our typical stripe. Hard to describe, alternating light and dark streaks.

 

[kenvlach]

Another rinsing problems is if the alkaline cleaner (silicated?) temperature is high and drying occurs while the parts are being withdrawn and transferred to the rinse station. Worse with heavier parts -- more thermal mass. Pretty certain you are aware of this, though.

Take a look at MIL-HDBK-509 CLEANING AND TREATMENT OF ALUMINUM PARTS PRIOR TO PAINTING.
This process uses both the Amchem 6-16 Deox. & Alodine 1200S. It incorporates an acidulated DEOXYLYTE final rinse, and also mentions

"For continuously high production lines, an Alodine 1200 toner or Alodine 45 coating chemical shall be added to the bath. The Amchem technical representative shall recommend the proper amount to be added."

Henkel Surface Technologies (Amchem's purchaser) still carries DEOXYLYTE products; not sure about the Alodine additives.