Replacing Electrodeposited Zinc with Phosphate Coating 1

[MechEng1977]

We are investigating replacing our Electrodeposited Zinc plated nuts and bolts with Phosphate + Oil due to the following reasons. Can someone please verify if these are true.

1)Phosphating is more cost effective? True or False?

2)Electrodeposited nuts and bolts will have a section on the part where there is no plating (the area they are hung)?
True or False?

3)Electrodeposited nuts and bolts are easily scratched and therefore will lose their coating protection whereas Phosphating chemcially alters the outer skin to provide improved protection. True or False?

Specifically, we have SC4, Type 1 and SC2, Type 1 Electrodeposited Zinc parts. What is the equivalent Phosphate Grade: OD, Type I?

Thanks in advance!

 

[CoryPad]

1) True

2) False

3) False

There is no phosphate coating that can provide equivalent corrosion protection to SC2 or SC4. If you want a better coating, go for multilayer zinc-rich coatings like those from Magni, Doerken or Atotech.

 

[MechEng1977]

If you read ASTM B633 the Min Salt Spray performance for Type II Electro-Zinc is 96 hrs.

For Phosphating as per ASTM 1137 Grade I it will achieve 168 hrs.

So I'm confused about your last sentence.

 

[CoryPad]

Phosphating according to ASTM F 1137 Grade I achieves its corrosion resistance due to barrier protection from zinc phosphate and the oil. Electroplated zinc provides corrosion resistance due to galvanic protection of the base steel. Neutral salt spray/fog testing according to ASTM B 117 has been found to have poor correlation to actual fastener usage. You are better off using zinc than zinc phosphate because the oil is not permanent and provides much of the supposed corrosion protection. My recommended coatings are of the Grade II and Grade III types identified in ASTM F 1137. You will find the commercial situation is advantageous when considering these coating types, especially if your part volumes are high.

 

[KENAT]

To back up Cory, when we looked at a similar issue about 10 years ago, I seem to recall parts protected by phosphate + oil weren't as mechanically durable. If you look at phosphate screws that have been used and in place for a while, the heads often show corrosion where drivers may have damaged the coating.

KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at posting policies:

http://eng-tips.com/market.cfm?

What is Engineering anyway: faq1088-1484

 

[MechEng1977]

Thanks for the responses.

Does anybody know if Type I and II require under-oversizing of the threads? I know the Type OD does not.

 

[CoryPad]

Type I or Type II from ASTM B 633? No, they don't need to be under or oversized.

 

[MechEng1977]

No Type I or II from ASTM B1137 Phosphated. Do they have to be under/over sized?

 

[CoryPad]

You mean Grade I and Grade II from ASTM F 1137? If so, then you don't need to worry about that unless the thread size is smaller than M8 (5/16"). If you do need to adjust the thread size, the ASTM standard states that this is permissible.