electroless nickel plating over welded mild steel? 1

[stonerabroad]

we've just welded up some mild steel flanges (1018/1020) to an A53 pipe, then electroless nickel plated the assembly.

small cracks have been noted in the final piece and questions are arising as to whether it was the welding process that created the cracks, or the post-weld plating.

has anyone ever experienced weld cracking as a result of applying electroless nickel?

these parts were heat treated to stress relieve the welds prior to nickel plating.

CS

 

[HardMetal]

Are the cracks in the coating or in the base metal? How did you detect the cracks? Did you do any NDE prior to coating?

Paul

 

[HardMetal]

What thickness coating did you apply? The coating may crack if it's too thick.

 

[kenvlach]

Cracking of electroless nickel is extremely rare (couldn’t pass the 100hr salt spray test!); I have never seen except in bent adhesion specimens. However, an extremely thick coating or an ‘out-of-balance’ plating or a quick cooling of really sharp geometries from 190-195 F plating temperature might conceivably do it.
I would also like answers to HardMetal’s questions.

From the present info, my guess is that there weld cracks which were slightly enlarged during acidic pretreatment prior to plating.* Due to residues in the crack, the electrolytic nickel ‘strike’ coating did not cover the crack. During the subsequent electroless nickel plating, at some point the EN bridged the crack via lateral growth. But, with no adhesion here, and good adhesion on both sides, any residual stress plus thermal contraction during rinsing cracked the EN coating to reveal the original crack.
*This might be a good point to check for cracks.

 

[HardMetal]

I would think that doing NDT of welds prior to plating would be standard operating procedure. You should institute a plan to do that.

 

[stonerabroad]

the small cracks noted were through the welds. hydrostatic pressure testing of the welded assembly prior to plating was done and passed. a second pressure test was done after plating and cracks through the welds were noted.

as a result, the part failed. and simply due to the circumstances it appears the cracks through the welds occurred as a result of the electroless nickel plating process.

note that it is the welds that have cracked. the coating thickness is only 6 to 8 tenths of a mil.
the acid pretreatment is only 10 seconds, to simply activate the surface prior to plating.

 

[ornerynorsk]

I'm not a metallurgist . . . . would hydrogen embrittlement come into play in this instance? Have you had the platers "bake" your weldments after plating? After all, even mild steel will "harden up" in the heat affected zone.

 

[kenvlach]

The stresses in EN coatings are normally very low, see

http://www.macindustrialproducts.com/properties.html,

and your coating is less than a mil, so I don't believe it is the source of your problem.

Hydrogen embrittlement is not involved unless the hydrogen entered during welding and had an effect during cooling. The stress relief would have baked-out any hydrogen, and the subsequent processes as-described are far too mild to have caused an HE problem.

My earlier comment re checking for cracks was because I think that there may be closed cracks from welding, not detected by present methods, that are highlighted by acidic cleaning, etc. Agree with Hardmetal re NDT, and suggest you use more sensitive methods than hydrostatic testing.

 

[HardMetal]

Stoner,

What pressure do you pressurized the part to? How long do you hold it at that pressure? You should have a specification for testing the welded joint.

 

[stonerabroad]

as for EN process conditions, the plating bath was at 165F and the rinse at around 100F. the parts were allowed to cool to room temperature after that.

as for the welding, a standard was used that required the parts to be preheated only to 70F (note that i'm emailing from canada and shop temperatures these days are in the 40's). an experienced welder did do the welding.

also note that there was no stress relief after welding. i don't know what pressure the hydrostatic pressure testing was done at but will know shortly.

these parts have been fabricated for some time from the same steels, but had been cadmium electroplated for corrosion resistance in the past. with cad plating not preferred in european markets, we've moved to electroless nickel. the parts manufactured with the cad plating have never had any weld problems. now that we have noted them one of two things will happen.

1. all welds will be designed out of the parts to eliminate the possibility of weld related problems.
2. parts will be sectioned to determine the cause of this weld cracking. parts already in the field may be recalled.

electroless nickel is an excellent coating and i wanted ensure that this coating, or the process by which it was deposited, was not the cause of any weld cracking.

does anyone believe that for any reason electroless nickel would result in cracking where electroplated cadmium would not result in cracking.

thanks. kenvlach, your comments agree with my original assumptions.

 

[kenvlach]

stoner,
It’s too bad that your company felt it necessary to condemn welding; somebody let you down by not following proper procedure (possibly no preheat, probably no stress relief, limited testing, etc.).

Your remark about the ambient shop temperature reminded me of another possible problem. Welding electrodes (at least coated ones) are either stored in original, hermetically sealed packaging or in drying ovens. It’s possible for electrodes to absorb moisture otherwise, which then enters the weld metal as hydrogen. If oxy-acetylene were used instead of arc welding, improper flame mixture maybe was a problem.

Suggest you make sure that the analysis of sectioned samples is very thorough; do hardness measurements across the cross section in addition to metallography, etc.

I hope you get a definitive answer and let us know of the outcome (I don’t like aspersions cast against EN!).

Ken V.

 

[HardMetal]

I've never heard of EN causing cracking in welds. As is often the case in production, when one thing changes (the plating process) something else changes (usually something one was not aware of). There are many things to consider. How do you measure your temperature? Are you sure it was 70F? That sounds kind of low for a preheat temp.

Are the parts being cleaned the same way for the EN as for the Cd plating?

Have you checked the chemistry of the EN? Is it correct? Are you sure it's not electroplated nickel?

Paul

 

[stonerabroad]

as an update for everyone following this thread, the parts have been sectioned and a little more information obtained from the fabricator.

the cracked weld was a result of
1. poor set up, a third spacer ring was created by the fabricator and not fully penetrated during welding
2. no preheat was used prior to welding
3. no stress relief was done after welding
4. pressure testing was apparently done after electroless nickel plating. upon notice of the leaks, no preheat was applied and a high carbon nickel electrode was used to patch the void in the welds. the lack of preheat resulted in hot cracking and the phosphorus in the electroless nickel created voids at the weld material/base material interface.

to finish up, we have suspended operations with the shop we're dealing with, and have decided to specify full penetration welds and weld procedures for any welded parts. in addition, we are specifying a stress relief prior to plating.

any comments. thanks for everyone's help on this.