Weld-able Primer Concerns

[fabengr]

We have been asked to provide structural steel that is shop painted using a verity of shop coatings.
In places where the field will be placing welds we have been asked to coat the weld hold back area with a weld-able primer.
I have always been concerned that there cannot be anything in the weld / HAZ when the weld is made.
I want to push back against the requirement on the grounds that the weld-able primer will add "stuff" to the weld metal that is not part of the weld metal design and thereby possibly change the chemistry to something different that what is called for in the design of the weld metal / base metal.
Is my concern justified?

 

[DVWE]

Sounds to me like you're between a rock and a hard place.

Your customer is most likely requesting the weldable primer as a means of protecting the weld prepped ends from normal field conditions, such as rain, mud, etc. It is a valid request, especially considering the length of time some structural components can sit in the weather prior to installation. To me, that's your biggest hurdle to convince them of how this will be controlled otherwise. If there are many field welds, there will be a lot of time before the last few get done. Rust and field grime are bigger problems, how will you handle them?

Manufacturers of weldable primer claim the primer does not have to be removed, and that it actually "improves weld quality". I've never been a proponent of that claim, but I have no data to base my position.

Our field crews are instructed to remove any paint, including weldable primer, before welding. It does not take much extra effort to remove. The primer absolutely helps to protect the weld prepped ends from normal and harsh field conditions.

Your concern is valid as well, but your customer will probably not buy into "adds 'stuff' that possibly will change the chemistry of the weld." If you're truly concerned, give them some valuable data to consider. Weld a test coupon and have a chemical analysis done on it. Tell them the welder's perspective on how it changes the weldability. Perform some bend tests. Give them something. Ensure them that all rust and grime will be removed prior to welding, the weld joints will be adequately cleaned, and the weldable primer is not necessary.

Will it cost time and money? Yes, of course. However, depending on the amount and timing of the field welds, you might be time and money ahead to perform the tests...

 

[weldstan]

In the late 1970's, we tested welds made into weldable primers for use at the North Slope. All of our tests on primers, properly applied, were acceptable in meeting tensile and -50 F impact properties and radiographic quality. Comparisons with welds made on clean surfaces showed no real differences.

One caveat is "properly applied". We have seen primers with thickness much over specification, especially the zinc rich types, and radiographic quality was often a problem. As a result we often required the primer to be fully removed by grinding prior to field welding.

 

[Mr168]

We've also had significant quality issues when welding over machined surfaces that had primer applied prior to PWHT to prevent scaling, particularly when using GTAW.

I will second weldstan's comment on coating thickness. I've seen more than my fair share of bevels that have the primer slathered on with a paintbrush, bristles and all trapped in about a 1/16" thick layer of "glopped on grey".

 

[gtaw]

There is a weldable primer called "Bloxide". It is an aluminum based paint that can be used to provide temporary protecting on surfaces that will be welded.

The company I worked for would spray it on the surfaces that were to be field welded. We would wire brush the joint before welding. Wire brushing would remove most of the paint and the underlying surfaces would be free of rust and ready to weld. Some welders would weld over the paint, but most would wire brush the joints first. The welders that didn't wire brush the joints didn't fare well, but I don't believe it had anything to do with the paint, it was more of a problem with attitude and welding skill.

Best regards - Al