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


-----

Adhesion problem CARC painting zinc plated hydraulic pipes

We design & manufacture military bridging systems here in the UK for the MoD and US military. We are experiencing great difficulties in getting the WD CARC paint to adhere to the hydraulic pipe work. We currently degrease, undercoat and top coat. We do not etch primer for fear of compromising the passivated coating. We have also tried degreasing and top coat only. We have dicussed this issue with our paint suppliers but they tell us that you cannot successfully paint a passivated surface unless etch primer is applied. Has anybody come across this issue? We also have the same problem with plastic components.

Phil Brownlow
manufacturing Engineer - Stockport, UK
August 13, 2008

---

Hi, Phil. This is a major issue and a recent one. Before the days of RoHS, chromating was hexavalent and virtually generic. Adhesion problems only occurred if the chromating was too heavy or defective in some fashion. Since RoHS, chromating is trivalent, and to achieve acceptable corrosion resistance top coats of various types are often used; plus there are thick film chromates, and thin film chromates, and other variations. Some are readily paintable, some aren't, and worst of all (from your standpoint) it's all proprietary. I'll give you 5:1 the problem is in the plating shop not your shop, and it will be very hard for you to fix. And no sooner will you, then the plating shop will change suppliers or the OEM will go to a different plating shop and you'll be back at square one.

You have to explain the problem to the OEM and figure out what to do.

Regards,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
August 13, 2008

---

August 15, 2008

Phil,

What is the material that you are working with? It sounds to me like passivated stainless steel. If so I think your paint suppliers are probably right. Can you use a BS X33 Type II primer? This is the high adhesion primer, it might give you a fighting chance.

Otherwise do as Ted says and go back to the OEM and explain the situation. They may be able to change the treatments to make it more farourable for paint adhesion.

If the OEM states that it must be passivated then you really have only one option if the BS X33 doesn't work and that is the etch prime route.

Brian Terry
Aerospace - Yeovil, Somerset, UK