Poor durability of black oxide on 17-4PH stainless 1

[tbuelna]

I have some parts made from 17-4PH cond H1025 stainless steel that require a black surface finish for cosmetic purposes only. I have been using MIL-DTL-13924 class 4 black oxide, but this black oxide coating is very easy to damage. I appreciate that black oxide coating is not intended to provide corrosion protection or wear resistance, but this black oxide comes off just from the rubbing contact experienced during routine handling. I did a comparison test with the black oxide coating on some commercial alloy steel fasteners I had. I rubbed both samples with fine steel wool (dry) and the black oxide coating on the fasteners was significantly more durable than the black oxide on my stainless parts.

Can anyone think of a reason for this situation? Am I using the proper type of black oxide process for my 17-4PH material? Or does this particular type of black oxide just have very poor durability?

Thanks for any help.

 

[CoryPad]

Terry,

I remember your previous thread on this subject (thread723-366531), and my second post mentioned an active surface. I believe that is your issue. For black oxide to work well, the stainless steel surface needs to be active. Here is one description of a preferred process:

To activate stainless steel before oxidizing requires a 50% sulfuric or hydrochloric acid solution. This activates the surface of the part and prepares the material surface to allow the black oxide to penetrate the stainless steel surface.

Talk to your supplier about activation prior to the black oxide immersion step. Good luck.

 

[tbuelna]

CoryPad-

Thanks for the input. I should have taken the time to go back and carefully re-read the responses on that thread. My current question is related to a processing problem rather than seeking advice on selecting a particular process, so I thought it might warrant a new thread.

I am using MIL-DTL-13924D and I read thru the spec to see if there is any mention of activation of a cres surface prior to oxidizing. I didn't see any specific mention of it in MIL-DTL-13924, and sec 3.2 "Preparation of basis metal" only describes a thorough cleaning and stress relief. After rinsing, classes 1,2,3 require a 30sec dip in a 0.06% chromic acid solution followed by air drying. But this does not sound similar to the activation process you noted.

Sec 6.3.2 "Processing" states vendors may use proprietary processes but the processing details shall conform to MIL-HDBK-205. I got a copy of that and I will read thru it to see if there is any mention of activating the surface of cres parts prior to oxidizing.

In your 2nd post of the related thread, you suggested asking the vendor "Can they treat a part that has been passivated?". I had the same vendor process two separate 100pc batches of these 17-4PH pins about 8 weeks apart, both to MIL-DTL-13924 class 4, and got similar results. The pins were 100% machined (every surface) from heat treated CD bar stock. They were not passivated after machining, and the only processing done between machining and delivery to the black oxide vendor was a vibratory tumble deburr using a water/soap solution to keep the media clean. I am currently machining another batch of the same pins, but before black oxide coating I will talk to the vendor about activating the material surface after cleaning but prior to oxidizing, as you suggest.

My knowledge of chemistry is fairly limited, so your help is greatly appreciated.

 

[CoryPad]

Definitely worth its own thread.

Take a look at Figure 11 in MIL-HDBK-205, especially the block called ACID PICKLE. That is the activation process.

 

[tbuelna]

Thanks CoryPad.

Looking at that flow chart in fig. 11 the acid pickle cleaning process is on a path parallel to two other cleaning processes, so I'm assuming the vendor has the option of using any of the three unless instructed otherwise. If the vendor has the option of selecting a cleaning process, I'm sure they will choose the quickest and least costly one, which is definitely not acid pickle.

Section 2.1.7 "Acid Pickling" describes this as a heavy-duty cleaning process that should only be used when other methods won't do a thorough job, and there is no mention of activating a passive cres surface. They point out that the acid pickling solutions will leave hydrogen in the material, so embrittlement relief should always be used.

The handbook also mentions that due to the aggressive nature of the acid solutions used for pickling you need to be very specific about describing the acid solution and process variables that shall be used, so that there won't be excessive material removed or surface pitting. I'll have to ask my vendor if they have the existing capability/expertise to perform acid pickling on my 17-4 cres parts. I'll have around $2K invested in the batch of parts I will be coating, but I'm thinking I might want to pay the $120 lot charge to have a couple sample parts processed to make sure everything comes out OK. My vendor is a reputable outfit, but they are very clear that they assume no liability for the finished product as long as they followed instructions on the PO.

This has been a great learning experience so far. And I'm sure I'll have more questions in the next couple weeks as I go forward with this project.

 

[tbuelna]

CoryPad-

A quick update. I've been continuing research with this black oxide on 17-4PH stainless problem. I found this company EPI which markets a proprietary black oxide process called Ultra-Blak 407 that conforms to MIL-DTL-13924 class 4. They sent the process instructions, and sure enough they describe a fairly detailed activation step just prior to oxidizing.

The process instructions also include this statement about excessive immersion time in the oxidizing solution causing non-adherence: "Required immersion times may vary from 2 to 15 minutes depending upon the mass of parts and the type and condition of the stainless steel. Excessive immersion times may lead to non-adherent finishes." So that's another thing I will have to check into.

 

[CoryPad]

Thanks for the update, that is in line with my previous comments.