Where the
world gathers for metal finishing
Q&As since 1989


-----

Appearance problem on Chemical Conversion Coating

My Alodine process(Alodine 1200S) consists of vapor degrease, alkaline clean, rinse, deoxidize, rinse, Alodine, and rinse. Precleaning process is being shared with other surface treatment (Chromic, Sulfuric acid Anodize).

My questions are the following :

1) These days, Alodine quality, especially appearance, has been terrible. Even though it is multi-color coating, it looks colorless coating after Alodine process. My deoxidize is nitric acid based deoxidizer. Some technical document says that nitric acid based deoxidizer may cause discolor problem in Alodine process. Is that true ?

2) If not, please tell me how to approach a real cause ?

Hopefully,

Dong-cheol, Park
- Busan, Kyung nam, South Korea
2003

---

What is the pH of the chromate? It is acting like it is too high. Dip out a bucket, lower pH 0.1 then 0.5 on paper, use nitric acid to lower the pH, see if that gives a yellow color. What is the alloy? Nitric Acid removes oxides from ONLY aluminum. Alloyed aluminum sometimes requires a mixed acid (proprietary) deoxidizer to remove the alloying ingredients from the surface.

Robert H Probert
Robert H Probert Technical Services

Garner, North Carolina

2003