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Gas Turbine Blade Refurbishing. Etching Question

In the refinishing/repair of gas turbine blades, is it necessary, or desirable for the acid (HCL) bath to etch the base metal of the blade prior to re-plating? The acid bath removes the nickel/chromium plating prior to re-plating. Is it required to remove the old plating down to base metal and etch the base metal to prepare for re-plating or is the etching of base metal not desirable?

Thanks for all replies.

William B [last name deleted for privacy by Editor]
- NY USA
2003

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Normally, you have no choice. You usually have to follow the engine manufacturers standard practices or have your own FAA in-house certified engineer.

Out of curiosity, which blades from what engine? I was not aware of any chrome plated blades, but I have not been associated with engines for 10 years or so .

James Watts
- Navarre, Florida
2003

This question might have two different and probably opposite points of view. From the point of view of the plate adhesion, some etching is desirable to maximize it. From the point of view of structural resistance it might be arguable depending on several factors (base material, strength design, cross section, present surface condition and other). I think that due to the critical application you have to strictly follow the manufactures specs or ask the customer's technician for instructions rather than an outsider opinion.

Guillermo Marrufo
Monterrey, NL, Mexico
2003

William, I am with [James] 110% Follow Mfg, or FAA. HCl can cause grain boundary attack and Hydrogen embrittlement. I have seen big engines that have come apart in service, not pretty and potential for really bad things. Remember the United DC-10 where the turbine blade failed and cut the hydraulics?

Jon Quirt
- Minneapolis, Minnesota
2003

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I'm an environmental regulator. I recently inspected a gas turbine blade repair facility. They subject the parts to an HCl bath to remove the old plating/coating and re-plate the parts with either Nickel or Platinum.

It is important from a regulatory perspective to determine whether it is necessary or beneficial to etch the base metal to prepare for re-plating. I don't have access to the OEM specifications. I understand that it is not desirable to remove too much of the base metal in order to maintain part integrity but I need to know whether some etching of the base would be necessary to accept the nickel plating. Thanks for the responses and interest.

William B [last name deleted for privacy by Editor]
- New Paltz NY USA
2003

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2003

Actually, I think the question was answered, Mr. Buskey, unless I'm not following you. If the spec says it's necessary, then it's necessary; if the spec doesn't call for etching, then it's not only unnecessary, it's not even permissible.

The engineering team who has accepted responsibility for the spec and the lives of the passengers are the only people who should hold any opinion on the question of whether etching is necessary. The rest of us must not try to decide what process steps are necessary based on general principals. The regulated facility should produce a copy of the process spec. for you so you can see whether or not it calls for etching. If the spec is not clear then we must be careful to not inadvertently impose pressure on the process engineer to skip a process step he felt was important. Sorry if I am misunderstanding.

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

We are not trying to pressure the company to do anything related to their process. We are trying to make a hazardous waste determination for the wastewater treatment sludge where it is important to determine whether their coating stripping operation is also an etching operation. We got conficting information during the inspection where some employees indicated the base metal was etched and company mgt stating it wasn't. I was trying to find out if there is an industry standard. I think we will ask the company to produce the OEM specifications. I thank you for your comments.

William B [last name deleted for privacy by Editor]
- New Paltz
2003

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Yes, it certainly sounds like the right approach to tell them they must produce the spec so you see for yourself what it calls for.

Regards,

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