ABSTRACT

The nut and bolt thread protector system is a patented, ultraviolet stabilized polyethylene thread protector which seals nut and bolt threads in pressurized grease to effectively prevent corrosion and provide protection against physical damage. Laboratory corrosion testing has shown no corrosion on bolting using this system in 90-day ASTM B 117 salt spray tests and in 60-day wet sour gas tests. The system has proven its effectiveness for the corrosion protection of bolting in many marine applications. The corrosion protection provided allows removing the nut from the bolt after long exposure in corrosive environments without the use of special tools, and dramatically extends the useful life of the nut and bolt. The result is the elimination change-outs required from corrosion damage and significantly easier change outs when they are required for operational reasons, providing significant time savings for maintenance personnel. The system is state-of-the-art in corrosion protection for bolting in corrosive marine environments.

INTRODUCTION

Corrosion of bolting is a very old problem, and has existed since nuts and bolts were first used to hold two things together. Bolting is often the "weak link" in the joint from a corrosion standpoint. However, corrosion of bolting is often taken for granted and not fully considered in the standard corrosion protection specifications for a facility. Much more attention is paid to the structure, vessel or piping materials than to what is actually holding the facility together. Corroded bolting can severely compromise the integrity of a facility, and have resulted in failures. Bolting is a real life example of "the devil is in the details."

Corrosion of the bolt threads results in a significantly roughened surface on both the exposed threads and the threads covered by the nut. The corrosion t)roduct formed takes up more volume than the metal This fills the thread area between the bolt and nut and increases the load on the bolt.* The result is a weakened joint from loss of metal and increased bolt loading, and increased difficulty or inability to remove the nut from the bolt. The former compromises the integrity of the joint, while the latter makes removal and replacement of the bolts extremely difficult.

The marine atmosphere is highly corrosive and is characterized by fine wind-swept sodium chloride particles that deposit on surfaces. 2 These air-borne salts (chlorides) deposit on bolting, and with alternate wetting and drying the chlorides can concentrate providing a very corrosive condition. The chlorides are often hygroscopic which increases the time of wetness resulting in increased corrosion. 3

A number of protection systems to mitigate bolting corrosion have been employed to counter these problems. These systems fall into the broad categories of coatings or corrosion resistant alloys. Coatings include paints, grease or wax, fluoropolymers (PTFE), electroplating (Cd, Zn, Zn-Ni, Cr, and Ni), and hot-dip galvanizing. Corrosion resistant alloy bolting materials are available to resist severe environments. These protection methods are compared in Table 1.

Table 1. Comparison of commonly used methods to prevent bolting corrosion.

Coatings

A coating attempts to prevent corrosion of the bolting by placing a thin physical barrier between the bolting and the corrosive environment, or in some cases to provide chemical or galvanic corrosion inhibition.

Painting bolting is an inexpensive protection method, both in terms of installation and use. This method of protection is not ideal because the coating is easily damaged or can simply wear away with time, exposing bare metal. Corrosion readily occurs at s

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