Stainless Steel or Zinc plated Screw 1

[tmech77]

Situation #1: I have two powder coated steel plates that will be attached with #8-32 screws. There are clearance holes in one plate and threads in the other plate. They will be installed indoors at room temperature in Florida. I am concerned with the high concentration of salt in the moist humid air so I want the correct screw material to reduce the possibility of galvanic corrosion between the screws and the steel plates. Cost and strength are not a consideration. Would electrodeposited zinc plated steel screws or 316 passivated stainless steel screws be a better choice to avoid galvanic corrosion?

Situation #2: I am attaching a zinc plated steel DIN rail to one of the painted steel plates. What would be the correct choice screw material to best prevent corrosion between the screw and the DIN rail, an electrodeposited zinc plated screw or a stainless steel 316 screw?

Situation #3: If you specify a zinc plated screw for either of the above situations, is there a possibility that the zinc plating itself may eventually disappear and then the steel screw would start to rust?

 

[MikeHalloran]

How far from the beach?
How long do you want it to last?
Are there actual electrical requirements involved, e.g. do the plates have to bonded to each other electrically, independent of attaching means?

I think the tapped hole in the mild steel plate will disappear first.



Mike Halloran
Pembroke Pines, FL, USA

 

[tmech77]

Equipment will be 1 1/2 miles from the beach. It needs to last about 15 years. Plates do not need to be electrically bonded.

 

[MikeHalloran]

For 15 years, both screws proposed would be equally effective, except if circumstances produce localized corrosion cells in high humidity. Powdercoat both sides with drilled and/or tapped holes could catalyze localized corrosion, depending on geometry so far unrevealed.

If you're really worried about corrosion, and bonding is not required, nylon screws in tapped metal holes might be better, and ty-raps threaded through adjacent holes punched before powdercoating would be even better than that.



Mike Halloran
Pembroke Pines, FL, USA

 

[tbuelna]

Powder coating will not provide corrosion protection of threads in an #8-32 tapped hole. The powder coated bearing surfaces under the screw heads will likely also suffer some damage when the screws are installed.

 

[CoryPad]

Does the joint need to be serviceable after 15 years? Basic electroplated zinc plus passivation will not protect the steel screws for that duration, you will see steel corrosion products.

Stainless steel fasteners are not suitable for these parts, they will make galvanic corrosion worse.

A better option for these conditions is a steel fastener with a zinc-rich coating like Magni 565 or Dorken Delta-Protekt or Atotech Zintek.