Zinc plating, Mechanical galvanizing vs Hot-dip galvanizing 1

[Xianglu]

Would anyone provide some information of the process and main differences between Zinc plating, Mechanical galvanizing and Hot-dip galvanizing please? Particularly in the application of B7 bolts.

Thank you in advance.

Henry

 

[kenvlach]

B7 bolts (I am presuming per ASTM A193) should not be zinc coated. Zinc can embrittle alloy steel & stainless when heated. SS that has contacted zinc should be passivated in nitric acid before using at elevated temperature.

Also, for fasteners in general, any plating or coating may only be done by the manufacturer, who takes into account the effect of coating on thread tolerances.

 

[Xianglu]

Thank you Kenvlach!

Our customer requested that "High strength bolting (>150,000 psi [1100 MPa] ultimate tensile strength) shall not be hot-dip galvanized, but shall be coated by mechanical galvanizing.
We are in the situation that we have specified bolts as Zinc plated and the bolts have been purchased. (Some bolts are SA-320 L7). Our customer now requesting that all bolts shall not be Galvanized. My question is: Is zinc plating different from gavanizing and can we use the zinc plated L7 bolting?

Thanks again.

Henry

 

[kenvlach]

Zinc plated is vague -- means electroplated?

Zinc electroplating is usually prohibited for high strength steels (TS > 150 ksi) due to danger of hydrogen embrittlement. Medium strength steels (100-150 ksi) may require pre-plating stress relief & post-plating hydrogen bake-out. Zinc electroplating should be per ASTM F1941.

ASTM B695 is a spec. for mechanically plated zinc. It also has stress relief requirements for steels of TS > 1000 MPa.

Hot-dip galvanizing may be prohibited, as by your customer, due to hydrogen embrittlement in the acid pickling process, or thermal stresses when immersing into the molten zinc, but most probably, because the high temperature may soften the steel (additional tempering). Hot-dip galvanizing of fasteners may only be done in accordance with ASTM A153.

I cannot say whether your existing, plated SA-320 L7 bolts can be used. I see no language in ASTM A320 regarding plating, you must check the ASME SA320 spec. To convince your customer, do the A320 mechanical tests (Charpy impact at -101 ^oC & tensile test) plus hydrogen embrittlement testing for coated fasteners per ASTM F606.

 

[Xianglu]

Many Thanks, Kenvlach