1. Core design - The core, usually a pipe, has to be able to keep the anode reliably attached to the structure. The design will normally be reviewed by civil engineers to ensure that the strength of the attachment is sufficient for the weight of the anode as well as the projected launch forces and in-service hydrodynamic loads. The location of the core within the casting is also important because it will affect anode utilization factors used in the design calculations.

2. Driving potential - Clearly, the operating potential of the anode directly affects how much current the anode is capable of delivering to the cathodic protection effort. If there is a significant shortfall in the actual potential of the anode alloy, then there could be a major deficit of available current for polarization. This could lead to an under-protected structure or a poorly polarized structure.

3. Galvanic efficiency (capacity) - This value is critical in the design process since it dictates how many ampere-hours of protective current will be available for each unit-weight of anode material consumed. This is a very important property which should command high priority in the quality assurance process.

4. Weight - Obviously, the weight of the unit anodes is important since it translates directly to design life of the cathodic protection systems. While some tolerance exists, it is important to monitor weight very carefully.

5. Dimensions - The final dimensions of the produced anode are important, especially the length. The dimensions of the anode govern its electrical resistance to the seawater, length of course being the MOST critical dimension. Again there is some tolerance here, but since the dimensions also control the anode weight they are somewhat self-governing.

6. Casting integrity - The cathodic protection system designer needs to know that the anode utilization will not be affected by large chunks of anode material falling off prematurely. This can be prevented by core design and by control of major cracking of the casings. Many anode specifications pay (in the writer's opinion) too much attention to cracks, where common sense often reveals cracks to be much less important.

1. Core design - Bracelet anodes are manufactured in two basic configurations. The segmented bracelet is comprised of a number of small segments which are welded to hoops attached to the pipeline. The semi-cylindrical or half-shell anode is comprised of two pieces with integral cores which can be welded or bolted together onto the pipeline. The material quality and the location of the cores for these anode types is critical to utilizing anode efficiency and also in some cases because of the abuse that can be sustained during the pipeline installation.

2. Anode coatings - Unlike platform anodes, pipeline bracelets, like any other close or flush-mounted anode require a dielectric coating between the anode surface and the cathode to which it is attached. This is to prevent corrosion of the anode in the confined space which could build tremendous pressures with the formation of corrosion bi-products. It is also common to coat the supporting cores, especially on segmented bracelets. Since all underwater / subsea pipelines are coated, if the anode core were left bare it would represent a fairly significant holiday. Failure or omission of these coatings could lead to major problems with the cathodic protection system down the road.

3. Anode attachment - Because it is not common practice to weld the anode cores to the pipeline, the electrical attachment is usually made by a pair of cables. Better specifications call for solid welded or bolted bars. In either case, this is a weak point in the system. When cables are used it is common practice to powder weld the cables to both the anode core and the pipeline; this requires very close attention. The loss of this electrical connection will of course render the anode useless. The writer has experienced many cases where failure at this connection has led to failure of the cathodic protection system.

4. Dimensions - Clearly the dimensional tolerances are far tighter on a bracelet anode, which must fit snugly onto a pipeline.

Inspection and quality control requirements 

This section is not intended to be a specification. As previously mentioned, most major purchasers of aluminum anodes have their own detailed specifications; this section will address some of the more critical testing and inspection requirements. The information contained in this section represents the writer's experience, during which time over 32,000 anode assemblies have been inspected with a net weight in excess of 13,000,000 pounds.

Anode cores

Platform anodes - The pipe core of a platform anode should be of the specified type and size. Normally, platform anodes will have either a 2, 3, 4 or 6 inch schedule 80 pipe core. Specifications vary between seamless or ERW pipe, but the normal grade specified is ASTM A36 Grade B. It is a simple matter to check mill certificates and verify that the pipe joints supplied are from the heats indicated. It is important that the material is of the correct grade; otherwise the quality of the attachment weld could be compromised. Pipe which does not have original legible mill markings should be rejected. It makes sense to schedule these inspections at the pipe supplier's yard. Certainly the inspections should be completed prior to the cutting of the pipe.

Most platform anodes, except those designed for retrofit, will have bent cores designed to stand the anode off from the structure. The cores will either be best at 90° or 45°. Most specifications will address core centrality within the anode casting and will also address the parallel nature of the standoff legs. Both of these parameters can be easily checked with simple measuring jigs. From a practical and economic standpoint, the anodes should be rejected if the core is so uneven that it will present major problems to the installers who must weld the anode in place. The cost of major adjustments in the fabrication yard can amount to large expenditures rapidly.

Most specifications require that the cores be sandblasted to near white metal prior to casting of the anode. They also usually give elapsed-time limitations between blasting and the pouring of the anodes. These steps are easy to monitor. Many specifications call for a resistance check between the anode metal and the core, usually calling for a high-voltage earth tester. This test is totally unnecessary in the writer's opinion.

Pipeline bracelet anodes - Cores are usually fabricated from bar or rod stock. It is much more difficult to monitor the origin of these materials. The grade of material and its traceability are not as critical on bracelet anodes as long as the metal can be welded predictably. It is a good idea to randomly sample core material and assay by optical emission to check the chemical composition.

The most critical areas associated with the anode cores are the position of the core within the anode and the overall finished dimensions. A COMMON problem to be avoided is allowing the core to be exposed on the back face of the bracelet. The cores are usually by definition close to the back face, but should have the specified minimum cover of aluminum. An exposed core in this area can lead to corrosion product formation at coating defects on the anode; exposed core should cause an anode to be rejected. A simple template should be used to check overall dimensions and fit to the pipe.

Physical inspections

The physical inspections include weight and dimension monitoring, the quality of the casting, and the fabrication of the anodes. With pipeline anodes it also involves the coating of the anode and core bars. Packing and making are also included in this area. Specifications on weight and dimensions are normally easy to interpret, and consequently are easy to inspect and monitor. However, the specifications on casting defects are not well-defined in many cases. The propensity for an anode to crack or shrink depends on several factors. They all relate to the size of the casting with respect to cross section, length, core size and the composition of the specific anode. Most standard-sized anodes can be poured by experienced producers with little or no cracking / shrinkage. If some cracking does occur, it is unlikely to affect the long term performance of the anode. The writer would recommend rejecting anodes purely based on casting defects, only if the defect is one of the following:

Bracelet anode coatings will normally have a specified dry-film thickness range; this is very difficult to control with the type of coatings involved. The thickness gauges can be moved around to show virtually any value. The important issue is that the coating is applied as evenly as possible onto a well-prepared surface and that there are no visible holidays.

Anode markings should be given close attention. Sometimes, anodes have been shipped before all the long-term test results have been analyzed. In this case, anode heats which fail may have to be identified or even removed from the structure.

Chemical analysis

The chemical composition of the aluminum alloy used to create the anode is obviously critical to its performance. Specifications will always be in place to govern the ranges of the various elements making up the anode alloy. The normal methods used to assay the samples are atomic absorption, optical emission spectroscopy or inductively coupled plasma spectroscopy. Whichever method is used, there are some common procedures which should be adhered to:

Electro-mechanical testing

NACE has a standard test method [1], which covers the standard methods for testing aluminum anodes. In this section, I will attempt to review those methods briefly and point out some areas which require special attention.

Galvanic efficiency testing

There are two basic methods used for determining of the galvanic efficiency of a material: weight loss and hydrogen evolution. It is a good idea to employ both tests since one is a relatively quick test and the other runs for a longer term and thus gives a more representative overview of the quality of the material.