Coating Material Selection for Bolts/Nuts

Corrosion of bolts and nuts has been always a major concern in many industries. Corrosion would cause many problems such as difficulties in bolt tightening/losing or even results in the bolt failure. In this article, it is tried to specify the corrosion prevention method for bolting materials used for piping, equipment and structures in different atmospheres except the contained fluid in piping or equipment.

There are 3 major methods to prevent corrosion of bolting materials as follows:

1-     Use of Corrosion Resistant Materials:

Using Corrosion Resistant Alloys (CRA) such as ASTM A193 B8/B8M/B8T,… bolts and A194 8A/8MA/8T, … nuts. This method although is a good way to eliminate corrosion, however it is normally used when the base material (such as flange) is made of CRA. It is not normally suggested to be used on materials such as carbon or alloy steel since it imposes cost to the project where there exist cheaper solutions besides some limitations in selecting the proper bolting material.

B8 Machine bolt

2-     Use of bolt caps (protection shields):

Bolt cap is proper device which can prevent corrosion. There are many good practices of using such caps. They are mostly used on equipment but can also be used for piping bolts. They are normally available from bolt size of ?” to 6”.

3-     Use of protective coatings:

The most important method in preventing bolt/nut corrosion is the application of corrosion resistant coatings. Different coatings can be used to prevent the corrosion. However, the most prevalent coatings can be classified as follows:

1.      Hot dip Galvanized coating

2.      Zinc plating

3.      Zinc/Aluminum coating

4.      Cadmium coating

5.      Chromate Coating

6.      Phosphate coating

7.      Fluoropolymer (PTFE) coating

8.      Ceramic coating

9.      Wax

1.      Hot dip Galvanized coating (ASTM A 153/ ISO 10684/ ISO 1461)

The coating provides excellent corrosion resistance for both onshore and offshore applications. It can be applied according to different standards such as ASTM A 153 according to which the average thickness of 43 microns for up to and including 3/8” bolts and 53 microns for over 3/8” can be achieved.

Hot dip galvanized bolt after 3 years of exposure to coastal areas

There are some limitations and problems in using hot dip galvanized coating as follows:

a.      Excess zinc

Removal of excess zinc by spinning the parts in a centrifuge, or brushing the threaded portion, or handling otherwise to remove the excess zinc Parts shall be spun immediately following removal from the galvanizing bath and quenched in water.

b.     Hydrogen embrittlement

Hydrogen has the potential of being introduced into the steel during acid pickling, prior to hot-dip galvanizing. For high strength fasteners (having a specified minimum product hardness of 33 HRC), there is a risk of internal hydrogen embrittlement. If required by the product standard or by the purchaser, mechanical descaling (with or without flash pickling) or baking shall be conducted to reduce the risk of internal hydrogen embrittlement. Baking shall be conducted after pickling and prior to hot-dip galvanizing.

c.      Reduction of bolt strength

Galvanizing carried out at a temperature above 800°F [425°C] can adversely affect the final mechanical properties of the fasteners. Therefore, the supplier of the fasteners submitting the product to the galvanizer shall be aware of the tempering temperature of the fasteners relative to the temperature of the galvanizing bath and the potential effect it may have on the product. When requested by the purchaser, the average galvanizing temperature that the fasteners will be subjected to shall be furnished.

2.      Zinc plating (ASTM B633)

This is one of the most popular coatings types as it provides excellent adhesion to the substrate and provides suitable corrosion resistance. It is normally applied according to ASTM B 633.

They are applied in 4 thickness classes as follows:

The SC 4 condition with min 25 microns thickness provides the best corrosion resistance. Chromate treatment can be used in addition to zinc plating to extend its service life.

Max. applicable temperature for zinc plating is 210 °C based on ASTM A 194 and shall not be used in higher temperatures.

Due to limited thickness the coating is not suitable for offshore applications. Also we have faced considerable corrosion in Coastal areas with thickness of 14 to 17 microns in less than a year. However zinc plated bolts with 25 microns and also chromate treated has shown good appearance without any sign of corrosion after at least 5 years.

The possibility of Hydrogen embrittlement for high strength steels can be prevented by applying the stress relief heat treatment.

Zinc plated bolts with 14 to 17 microns thickness after 1 year in coastal areas

Zinc plated bolts/nuts with min. 25 microns thickness plus chromate treatment after 2 years of exposure to coastal areas without any sign of corrosion

3.      Zinc/Aluminum coating (ASTM F 1136/ EN ISO 10683/ EN 13858)

Normally known under the registered trademarks of DACROMET and GEOMET. They can provide very good corrosion resistance, especially for grade No. 4. However, we have experienced considerable corrosion in less than 6 month in coastal areas with grade No. 2. They are not suggested for marine atmosphere.

Hydrogen embrittlement is possible if the bolts are acid pickled or phosphate before application of the coating.

Bolting with grade No. 2 Zinc aluminum coating after 6 month exposure to coastal areas

4.      Cadmium coating (ASTM B 766)

Provides very good corrosion resistance upto 160 °C. However it is use is not normally suggested because of the Toxicity of Cadmium. Therefore cadmium should not be used in contact with food containers. Also, it shall not be used in temperature above 225 °C of where the heat from brazing, soldering or welding may cause the formation of poisonous cadmium oxide vapors.

Cadmium coating is deposited in 4 thickness class as follows. It is due to mention that Cadmium coatings thicker than 12 μm are normally not economical. However we have experienced drastic corrosion with class 12 in coastal areas in less than 4 years.

Cadmium plated coating with 12 microns thickness after 4 years of exposure to coastal areas

The possibility of Hydrogen embrittlement for high strength steels can be prevented by applying the stress relief heat treatment.

5.      Chromate coating

Chromate is not normally used as a single bolt coating. It is mostly used in addition to other coatings as a finished layer such as zinc plating and cadmium coating to extend their life and improves their corrosion resistance. They are really effective to enhance the corrosion resistance. Their use as top coat over zinc plating enhance their lifetime in coastal area to a few years.

Zinc plated bolts/nuts with min. 25 microns thickness plus chromate treatment after 2 years of exposure to coastal areas without any sign of corrosion

6.      Phosphate coating (ASTM F1137)

Phosphates are normally used as a primer for subsequent coating such as PTFE as it can improve the adhesivity of the next layer. Phosphating is not a proper coating to prevent corrosion resistance if it is used alone as a single layer coating in corrosive atmospheres.

There are several types of phosphate coatings such zinc, iron, manganese and magnesium, among which magnesium phosphate has shown the best corrosion resistance. There are 4 grades of phosphate coatings with regards of the supplemental coating according to ASTM F1137.

Machine bolt with phosphate coating

7.      Fluoropolymer (PTFE) coating

The Fluoropolymer coating (PTFE) is normally applied over other coatings such as zinc, zinc nickel or phosphates. It provides excellent corrosion resistance in both offshore and onshore applications. But are not recommended for subsea applications. The coating can resist temperatures up to 260 °C. They are normally applied in the thickness range of 8 to 30 microns. some PTFE coatings can endure up to 4000 hours or even more in salt spray test.

They are normally known under the registered trademarks such as XYLAN (e.g. XYLAN 1070).

PTFE coated bolts/nuts

PTFE coated Bolting used Subsea (EEMUA 194)

8.      Ceramic coating (ASTM F 1428)

Ceramic coating is normally used where high temperature application is required. They have also shown good corrosion resistance in several atmospheres including coastal areas Specially when they are top coated.

Ceramic coated bolt/nut

Although we have experienced heavy corrosion with grade No. 1C in less than a year in coastal areas, but Grade No. 4B has shown excellent behavior.

Ceramic coated bolts/nuts after 1 year of exposure to coastal areas

Ceramic coatings are also known under registered trademarks such as SermaGard and XYLAN (e.g. SermaGard 1105) some of which can endure up to 4000 hours or even more in salt spray test. They can resist temperatures up to 645 °C or even more.

9.      Wax

Wax-based lubricants is normally used as supplementary coating to other coating or as temporary coating or for short term protection or maintenance purposes. They are also useful during bolt tightening due to their lubrication properties. Wax-Based lubricants are normally water soluble and can be easily washed off after installation. Adding dye to the wax to distinguish such bolts from untreated bolts is a good approach when applying such bolts.

Wax coated nut

Summary of the different coatings properties are mentioned in below table: