Offshore and marine operators, without exception, spend the largest part of their corrosion budgets on paint maintenance. The failure of a “well applied” paint system can be traced to a few basic failure mechanisms. Some of these mechanisms are addressed in this paper. If the solutions presented are applied, then paint systems will last much longer, and maintenance budgets can be diverted to other areas.
The systems presented are no substitute for poor, out of specification, surface preparation or application. The writer therefore, would always recommend using qualified paint inspectors during the initial coating of any marine structure. The mechanisms presented do not represent a complete list, but do reflect some of the more common problems.
Pipe flanges are a built-in failure point on any piping system exposed to a marine atmosphere. From day one, when the flange is initially assembled, coatings on the inside hidden surfaces of the flange are damaged by the fasteners which hold the flange together. Once a flange is assembled, there is no way to paint these inner surfaces, but salt water and free oxygen intrusion, ensure that corrosion is free to occur. The result is unsightly corrosion products leaching from the flange gap and dripping onto other coated surfaces [Fig. 1]. The ultimate result could be the failure of the joint from either fastener corrosion [Fig. 2], or gasket seal corrosion.
|Figure 1 - Typical Flange Corrosion||Figure 2 - Fastener Corrosion Can Result|
The problem, along with some of the many unsuccessful solutions is discussed in an earlier paper . One of the more widely adopted solutions has been the use of “Flange Protectors” [Fig. 3]. These devices are more likely to cause problems than to solve them. The device consists of a stainless steel band with a sealing gasket applied in tension around the flange face across the gap. The device has a grease nipple designed to allow a grease to be pumped into the void spaces. The problem is that the tightly fitting band creates a capillary sized crevice which cannot dry, the paint fails under the band [Fig. 4], the corrosion product distorts the band, [Fig. 5] allowing water inside. The grease evaporates and the flange is free to corrode. Perhaps the most disturbing point is the fact that everything is hidden from view. Other attempts at solving the problem have included the use of petrolatum tapes, caulks, and full face blocking seals, none of these provides a satisfactory answer.
|Figure 3 - Newly Installed Flange Protector (Note Early Crevice Corrosion)|
|Figure 4 - Crevice Corrosion Develops, Begins to Distort Band||Figure 5 - Band Eventually Fails Leaving a Badly Corroded Flange|
In order to arrive at the best solution it was first necessary to list the desired attributes of such a solution, they are:
i) Provide corrosion protection for the life of the paint system
ii) Be quick, simple, safe and inexpensive to apply
iii) Be easy to inspect, nothing hidden
In order to achieve this, some specific design features would have to ensure:
iv) Not a size specific solution
v) Materials should be environmentally safe, non-sheening
vi) Corrosion control should be possible even though no surface preparation at all is possible inside the flange
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.
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.
|Figure 6 - Painters Applying FLANGESEAL on an Offshore Platform|
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.
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.
|Figure 7 - Flange fasteners show early signs of paint failure within months|
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]
|Figure 8 - Rack of Pipe U-Bolts Exposed for 2500 Hrs. in Salt Fog Cabinet. From left to right:
Sealed Aluminum Ceramic, Sealed Aluminum Ceramic, Fluorocarbon (With Primer Vendor A), Fluorocarbon (With Primer Vendor A), Sealed Aluminum Ceramic, Sealed Aluminum Ceramic, Fluorocarbon (No Primer Vendor A), Fluorocarbon (No Primer Vendor A), Fluorocarbon (No Primer Vendor B), Hot Dip Galvanize
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.
|Figure 9 - HDPE Fastener Protector Cap|
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.
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 .
|Figure 10 - Pipes on Pipe Racks Invariably Have Crevice Corrosion Problems||Figure 11 - Pipe Saddles and Half-Saddles Create An Aggressive Crevice and Make Inspection / Maintenance Almost Impossible|
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].
|Figure 12 - Rubber Pads Don’t Help, See Crevice Corrosion (Inset)||Figure 13 - Contoured Fiberglass Pads Provide Tight Crevice if Improperly Sealed|
As with all problems, when arriving at the solution, first the desired solution attributes must be identified, in this case they are fairly simple:
i) Eliminate the crevice and the ability to hold water against the pipe surface and the corrosion will be eliminated.
ii) Make the solution simple, safe and cost effective to apply.
iii) The solution must allow easy inspection and maintenance.
iv) Do not develop a solution which is size specific.
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.
|Figure 14 - I-Rods Bolted to Upper Beam Surface Provide Excellent Crevice Control|
For easier installation, the rod can be incorporated into the U-Bolt assembly [Fig. 15]. Over 1000 platforms in the Gulf of Mexico now have I-Rod systems in place.
|Figure 15 - I-Rods Pre-Installed to U-Bolts (called Nu-Bolt)|
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.
|Figure 16 - The rust stains on this tank originate from only 2 sq.in. of corrosion on the upper rim. The stains are unsightly and will eventually casue the paint system to fail.|
|Figure 17 - Before and after using the product "Rig Restore."|
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.
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.
 J. Britton: Corrosion of Painted Pipe Flanges and a Method for Its Control NACE Materials Performance March 1997.
 J. Britton : Corrosion At Pipe Supports – A Nagging Problem Solved Pipeline & Gas Journal April 1998