Zn-Ni and Zn-flake coatings for Fasteners/Bolt & Nuts

When it comes to the coating system selection for bolt & nuts in petrochemical and refinery plants there are a wide variety of coating systems with a wide range of price available for which many design engineers may have not enough information to have the best choice.

As a basic criteria, bolt & nuts shall be coated, and uncoated or even only phosphated bolts shall not be installed on the process piping or equipment.

There are some perfect choices for bolt coating such as PTFE coating with the basic Zn-Ni precoat which may seems an expensive one for C3/C4/C5 atmospheres without marine characteristic.

The best choices in terms of price and availability for bolting coating in atmospheres other than C5M, are Zn-Ni and Zn-flake coating systems which can be used in operating temperatures lower than 80 C. Of course, HDG is also a well-known choice at this temperature range but designers should keep in mind that HDG shall not be used in structural 10.9 grade bolting material.

Zn-Ni which is an electrodeposited coating system is widely used on fasteners and as a general reference for this type of coating ASTM B841 shall be considered to select the relevant grade and corrosion resistance. Type D of this coating with SST resistance of more than 500 hours seems to be a good choice in petrochemical services with OP temp less than 80 C located in C3/C4/C5 areas.

Zn-flake coating type which is a not an electrically applied fastener coating is another choice which is also widely used on fasteners as a general choice. The reference standard for its selection and test is ISO 10683. The minimum acceptable thickness for this coating in petrochemicals is 10 micron which shall provide at least 960 h SST resistance in laburatory.