Electroless Nickel Plating Problems 1

[Maui]

We are resistance welding electroless nickel plated tungsten carbide tips onto an alloy steel backing material during the manufacture of one of our products. The carbide tips are cylindrical in shape, measuring approximately 1/8" long by 1/16" in diameter, and plated with a 3 to 10 micron thick layer of nickel in order to aid in the welding process. When the plating is properly applied, the welding process works well, and the product performance is very consistent. Several months ago, we first noticed that there was a sporadic problem with the integrity of the welds on specific shipments of these carbide cylinders. After welding was completed, there were areas on the plating where it had clearly outgassed - this could be seen on the surface as bubbles that formed beneath the nickel plating. We could make the problem disappear by simply taking that particular bottle of carbide tips out of the production line, and replacing it with another bottle from a different shipment (but from the same manufacturer). The problem would go away immediately without changing anything else in the welding setup or process. Lately we are finding that an increasing percentage of these carbide cylinders are not bonding properly to the backing material, and the problem has spread to all of the incoming carbide shipments. We need to identify the cause of the problem. Does anyone have any experience with this type of problem who could shed some light on it? My suspicion is that there is some form of contamination in the plating bath used by the supplier, and this contaminant is forming into a gas during welding. The supplier denies any such contamination is present, and their examination of their processing records and the offending carbides has not produced any significant findings. I have contacted several plating houses, and talked with their plating experts. I am in the process of sending representative samples to them for independant analysis. I have also researched the posts on this website for any pertinent information. Does this ring a bell with anyone?

Maui

 

[kenvlach]

Could be substrate porosity, the plating pretreatment, the plating itself, or something during welding. From the adhesion problem described in your earlier thread:
Carbide Plating Source
thread330-162628
the plating pretreatment is a prime suspect. Lots of ways to go wrong: Over-etching, under-etching, letting parts passivate prior to plating, skipping a strike plating.
I suggest first testing adhesion of the EN to the carbide (WC/Co?). Try the Thermal shock test (aka Blistering test) in ASTM B733, para. 9.4.3: Heat the coated part to 200^oC in an oven and quench in room temperature water. Examine at 4X magnification for signs of poor adhesion.
Trapped moisture in plated-over pores will create blisters. EN is good at bridging over areas w/o bonding, so a Wood's nickel strike may be necessary. Is one used?

Was the plating certified to some specification (ASTM B733, MIL-C-26074E)?
Was a post-plating bake performed to improve adhesion?

The plater should have records of monthly adhesion and salt spray tests, preferably with signatures from an outside, certified lab. Ask to see the past 7 years' results.

As to the EN coating itself, is this a low-, mid- or high-phosphorus EN? I believe that welding problems increase with the [P]. Note that metallic phosphorous makes fireworks if heated (welding or grinding) in the presence of oxygen. Also, the Ni-Ni₃P eutectic at 880°C and 11 wt%P may cause welding problems.

Impurities within the EN coating increase as the bath gets used & replenished, especially, the orthophosphite byproduct. This causes surface roughness, failed salt spray tests and a slower plating rate. Since EN solution is expensive, the platers use the baths as long as possible, filtering out the increasing amount of precipitates and increasing temperature and pH to maintain a good plating rate.
Some platers use newer baths for better customers paying for MIL-spec work, and older baths for cheapo commercial work.

I lack confidence in your plater. You can check for EN-platers in the MA, CT & RI area:

http://www.finishing.com/Shops/index1.html#newengland

 

[tomwalz]

I own a couple patents in this area and would be happy to help.

Tom Walz

Thomas J. Walz
Carbide Processors, Inc.

http://www.carbideprocessors.com

 

[Maui]

Ken and Tom, thank you both for your help.

Maui

 

[kenvlach]

Maui,
Any update or more details?

Found 3 articles on EN plating in Products Finishing Online that may help:

'Are You Realizing the True Value of SCRAP?' by Brad Durkin of MacDermid
[SCRAP = Substrate, Cleaning, Rinsing, Activation and Plating steps for electroless nickel]:

http://www.pfonline.com/articles/030004.html

Notice the improvised adhesion test: “a modified impact test. The customer version was not a high-tech test, but one that characterized the condition of the coating’s adhesion with good reproducibility. Bang, bang, clink, clink. The areas of impact were examined.”

'Basic Substrate Strategies and Approaches for Electroless Nickel,' by Brad Durkin et al. of MacDermid. Gives the surface pretreatments for plating for various substrates; like having 6 ASTM 'Standard Practice for Preparation of ___ for Electroplating.'

http://www.pfonline.com/articles/web0301.html

I think perhaps plating on cast iron would be relevant to your material situation. Between this & Tom Walz's info, you should have a detailed plating procedure. Now, just get the plater to follow it!

'Electroless Nickel Problems' in Plating Clinic, by Art Kushner.

http://www.pfonline.com/articles/clinics/0803cl_plate2.html

Brief, but mentions the necessary monitoring of chemistry & impurities & filtration.

For more EN articles, search the PF Online archives:

http://www.pfonline.com/Scripts/Arc...ics&qp=&Zone=&qt=+Electroless+Nickel&x=60&y=6

388 items!

 

[Maui]

Ken, I spoke with the supplier directly and asked them several questions based on your suggestions. The most surprising result I obtained from this conversation was that the nickel plating is nickel sulfate, not nickel phosphate. I believed that they were using electroless nickel, but it appears that they are using electrolytic nickel plating instead. Sorry for the error - I should have confirmed what type of plating process was being utilized prior to posting this thread. The supplier said that they do not use Woods nickel strike. They also confirmed that they do not perform a strike plating of any kind. They use a 5% sulfuric acid solution for cleaning the tungsten carbide cylinders prior to nickel plating, and they dry them in a perforated tumbling barrel heated to 90 degrees C. They do not perform a post-plating bake, and claim that they never have. But they did suggest that we bake a shipment of carbides at 200 degrees Celsius in one of our furnaces for 2 hours, and then attempt to weld them after this post-processing step. They believe that their tumbling operation after the final rinse may not remove all of the moisture from the carbide surfaces before plating, and this trapped moisture could result in the formation of blisters during welding. We agreed to try this, and we will also run the blister test that you suggested on the same lot of parts.

The plater does not perform any salt spray tests. For adhesion, they take a jeweler’s file and scratch the nickel plating on the surface of the carbide. If the plating doesn’t flake off or peel, then they consider the plating adhesion to be acceptable. They take approximately 100 carbides from a batch of 100,000 and visually examine them for surface imperfections. After this examination is completed, they mount them in a Bakelite mount in cross-section, and examine them for microstructural defects. If none are detected (and few ever are) then they ship them. It is obvious that we need to establish a set of quality control tests to their final inspection process to verify that the plated carbides meet our quality standards. We will also need to perform those same tests in-house to verify the results obtained by the supplier. I'll keep you updated on our progress. If the information that I've provided suggests that we should focus on a specific step or task, let me know what you think.

Maui