The Optimum Solution

In order to arrive at the best solution it was first necessary to list the desired attributes of such a solution, they are:

With all these requirements taken into consideration, the FLANGESEAL system was developed. The basic theory of operation is simple, all internal surfaces are coated with a rust-converting oxygen diffusion barrier. The coating material selected is a nonhardening wax based product which has the consistency of vaseline at temperatures below 150° F, but when heated the viscosity drops close to that of water. This gives the product excellent penetrating capabilities. The flange is temporarily dammed with a heavy tape, and the heated material injected into the flange at a low point, with a high vent. When completely flooded, excess material is sucked back out using the two way syringe injector [Fig. 6]. When completed, the flange is completely sealed from corrosive elements but is still fully inspectable.

Monitoring Systems

One preventative method which can detect dislocated anodes or significant coating damage is a method of field gradient monitoring. This system utilizes two-dimensional field gradient sensors (electrochemical reference electrodes) which are more commonly employed in the monitoring of cathodic protection systems. The electrode array is capable of detecting changes in eleclrical field gradient adjacent to the pipeline without having to actually contact or otherwise interfere with the pipeline. A newly installed anode passing the array will cause a rapid gradient change in the seawater around the pipeline which can be detected by this array and reported on a computerized chart recorder. The upset in the field gradient will only occur if the anode is in electrical contact with the pipeline. Uthe anode should become isolated or detached from the pipeline, it will pass the array and create no gradient upset Thus, the operator will know that the anode has become detached. Since coating defects also cause an upset in the field gradient strength around a pipeline, damage areas can also be detected.

The sealing process is not at all weather restricted and can therefore be completed in conditions where regular blasting and painting ahs to stop. The applied cost is only a couple of dollars per flange. If any of the product should leak into the water, there is no problem because the material is completely non-toxic and will not make a sheen.

Fastener Corrosion

Fasteners are extremely difficult to paint, and, are often the first items to show paint breakdown [Fig. 7]. A combination of sharp edges, tight crevices and normal mechanical interface with wrenches and other tools has made this one of industry’s most expensive and annoying corrosion problems.

Solutions

There are two basic approaches to solving this problem, and the best solution depends on whether a new structure or an existing structure is involved.

New Construction: The obvious answer for new construction is to either make the fastener from a material which is resistant to corrosion in the intended service, in some cases this makes sense but usually is cost prohibitive. Alternately, the fastener can be coated with a coating appropriate for the intended service. So what is the best coating? There will be ongoing debate on this point, however, based on the writers experience, in salt laden marine atmospheres, and considering the natural abuse to which fasteners are subjected, we would make the following general recommendation:

When located in generally dry conditions at least 50 feet above mean water level use hot dip galvanized fasteners. When located close than 50 feet to the water, or if routine wetting with seawater is expected, use a sealed aluminum ceramic fastener coating. Comparative tests performed by the writer’s company, show conclusively that these coatings out perform all others in this type of exposure, and at a cost which makes sense. [Fig. 8]

Retrofit (existing fasteners): For retrofit applications it is often not practical to change the fasteners. If this is the case, the solution can be found in fastener protector caps. There are a number of different types available but the best we have seen is one which actually engages the threads of the stud to secure the cap over the nut [Fig. 9]. When the cap is filled with a dimensionally stable inhibited wax material, protection is complete and long term. The caps do not fall off, yet can be removed if the fastener needs to be removed, and reused. Beware of protector caps that just push onto the nut, as they can and will just as easily fall off. Material selected should be stable in a marine atmosphere, not adversely affected by UV.

The cap pictured is manufactured from high density polyethylene and has raised internal ridges in the shank which give the cap a “self-tapping” attachment to the screw. The shank is filled with the inhibited compound in the factory and squeezes around the nut as the cap is screwed on.

Pipe Support Corrosion

Whenever a pipe or conduit is supported, it is necessary to effect a mechanical interface to the pipe. This interface is often a corrosion hot spot. There are dozens of different pipe support designs, some better than others but they often share a common problem – crevice formation. [Fig. 10], [Fig. 11].

The crevice formed provides a water trap, this places the pipe coating system in immersion service. The coating systems on above water piping are not usually designed for this type of service and will consequently soften and fail. Once the coating has failed corrosion can occur and, with coating undercut can quickly spread [Fig 12]. A previous paper describes this in more detail [2].

Contrary to popular belief, it is not the metal-to-metal contact that causes the problem but the formation of a tight, water retaining crevice which develops an oxygen concentration cell. This can be seen in some of the “solutions” which don’t work, but in fact make the problem worse. [Fig. 12], [Fig. 13].

Solution

As with all problems, when arriving at the solution, first the desired solution attributes must be identified, in this case they are fairly simple:

The simple solution which was arrived at solves all of the above. A high compressive strength cast thermoplastic material with a half-round configuration (I-Rod) is deployed between the pipe and the support [Fig. 14]. The crevice is eliminated, no water retention is possible, and the in place system is easy to inspect and maintain without removal. In addition the system is inexpensive and simple to install by even unskilled labor.

Rust Stain Control

Corrosion product staining is unsightly and deleterious to paint systems if uncontrolled [Fig. 16]. The problem usually arises from either a small defect in the paint system, or corrosion at a flange, fastener or pipe support dripping rust stains onto good paint. Often the problem is a result of an overboard line which is corroding internally and dropping rusty water onto good paint. However it gets there, it’s difficult to get off without using a cleaner which either damages the underlying paint, the environment or the poor guy applying it.

Rust Stain Source Sealing

In the example shown [Fig. 16], we can remove the unsightly rust stains, but unless we seal the source of the corrosion product we will be cleaning again in a month or so. The same product which is used in the FLANGESEAL system, “SNOKOTE”, is also available in a nitrogen propelled aerosol and is marketed as MP-2000. This product can be directly applied to the source of the stain and will seal it off for several years until a full re-paint is scheduled or until a maintenance spot blast/paint crew can get to it.

Summary

The half dozen solutions shown here have all proven themselves, primarily in the Offshore Oil & Gas industry. When used in tandem, they will significantly extend the life or any marine paint system above the waterline. For more information on any of these products or systems, contact us.

References

[1] J. Britton: Corrosion of Painted Pipe Flanges and a Method for Its Control NACE Materials Performance March 1997.

[2] J. Britton : Corrosion At Pipe Supports – A Nagging Problem Solved Pipeline & Gas Journal April 1998