48047

Worse adhesion of EN after chemical polishing C75 (high carbon steel) [Switzerland] 

March 5, 2008

Hi to all !
We are facing serious problems after chemical deburring of C75 steel parts. Unfortunately we have no current on the line, it's completely chemical. After deburring we have 4 rinses, first one is clear but all following ones show strong yellowish Fe(III) precipitations. Afterwards we dip in a strong alkaline cleaner with intensive ultrasonic in order to remove the carbon remaining on the surface. Proper rinsing is next, followed by a short acid activation. Also in the following rinses, we observe yellow to brownish precipitations on the tank walls. Finally we plate 5 Micron of mid phos EN. The deposit peels off. Other steel types work perfect, C75 without deburring step also. We observed that adhesion improves a little bit if the EN bath gets older (more mto). Can anybody give us a hint ? Thanks in advance and best regards to the interested community.

Helge Schuch
engineer - Buochs, Nidwalden, Switzerland

From the info given, I think that you are having a flash rust after the polishing tank. A rusted part will not plate well. PS, yellow would be FeII, not III. You need to work with the Mfg. to find out why this metal is rusting and what you can do to reduce the effect. I will guess that you will need to raise the first rinse pH above 7 and possibly add some sodium glutanate or similar material to prevent the rust.Your chemical debur vendor should have the answer to this.

You may have to use a stronger acid etch before plate. I would try to use warm rinses and would add a little ammonia to the final rinse before EN--Get it up to the 6-7-8 pH range. This will also help the EN kick off which may help.

James Watts
- FL

Surely sounds like a smut problem to me. If possible, try cleaning a part manually after the deburr cycle and after the activation, like scrubbing it with a piece of scotch-brite. Then see what happens with the adhesion of the plate on the scrubbed area.

Guillermo Marrufo
Monterrey, NL, Mexico

Hi to all, thanks for the friendly answers and good ideas. Indeed it was a corrosion problem caused by over-active C75 steel after chemical polishing. The material oxidised already in the cold rinses before EN. Problem was solved by making up a special (weak acidic) predip including some Hypo: using this solution as final activation we can directly immerse into EN without need of additional rinsing. Adhesion is nearly perfect now. Thanks again and have a good time... Helge

Helge Schuch
- Switzerland