Anode Manufacture

There are a number of standards used for anode manufacture for cathodic protection systems. Mostly these standards are derived from NACE standard SP0387. DNVGL-RP-B401 also derives from SP0387 but diverges slightly in the requirements for quality control. Anode manufacturer’s in Europe will typically state adherence to DNVGL-RP-B401 but as stated above the standards are largely equivalent.

The CivilWeb Cathodic Protection Design Spreadsheet calculates the compliance of a cathodic protection system using any type or composition of anodes. Anodes should be manufactured in accordance with DNVGL-RP-B401 or NACE standard SP0387 in order to ensure that the system will perform as intended.

When purchasing anodes the buying may request the following information to be provided by the supplier;

  • The anode material type and details of the precise composition. This should include what standards are adhered to and any special requirements for the specific anode composition
  • Details of anode dimensions, type, net mass, insert details and any special requirements for fixing the anodes to the structure
  • Any special requirements for the anode utilization factor based on the specific type of anode and the design of the inserts
  • Any special requirements for pre-production qualification testing or for manufacturing procedure specification
  • Any special requirements for frequency of dimensional testing, destructive testing and electrochemical testing during manufacture
  • Any special requirements for the management of non-conformities identified during the manufacture or testing process
  • Any requirements for anode traceability, for example between different batches or anodes manufactured at different sites
  • Any requirements for retaining anode material samples
  • Any special requirements for handling, storage or transportation of anodes
  • Any special requirements for documentation

Anode Manufacturing Procedure Specification

An anode Manufacturing Procedure Specification (MPS) shall be prepared by the supplier for any anode orders in excess of around 15,000kg, though an MPS may be requested for smaller volumes. The MPS shall include the following;

  • Specification of anode insert materials
  • Details on handling and storage of materials
  • Details on the precise composition of anodic materials including maximum contents for impurities
  • Detailed drawings of the anodes including details of the inserts and the anode mass and dimensions including the manufacturing tolerances
  • Welding procedure specification and reference to qualification test for any welding of the anode inserts, and qualification requirements for welders
  • Preparation requirements for anode inserts before and during casting
  • Anode casting procedures including any temperature controls and the addition of alloying elements
  • Inspection and testing requirements for the anodes during and after casting
  • Any required coatings for flush mounted anodes
  • Handling, storage and transportation requirements
  • Anode marks, tracing and documentation requirements
  • Any further information as requested by the purchaser

Anode Alloy Pouring

Over-pouring to fill shrinkage depressions shall be kept to a minimum. All pouring of molten anode alloy should be completed before the surface of the anode solidifies. This can be delayed by applying heat to the surface (from gas burners for example) to keep it in a liquid state until the molten anode pouring is complete. However, once the anode surface has solidified it should not be re-melted, not even to fill shrinkage depressions.

Surface Preparation Materials

Any materials used for surface preparation or coating of the anodes should be contained in their original packaging until use. They should be marked with the following information;

  • Manufacturer’s name
  • Location of manufacture
  • Material type and product designation
  • Batch number
  • Date of manufacturing and shelf life if applicable
  • Manufacture standard
  • Instructions for handling, storage and transportation
  • Health and safety notes and other relevant info

Surface preparation materials must be stored, handled and transported in accordance with the manufacturer’s instructions. This will avoid any potential damage by the environment or other effects.

Coating of flush mounted anodes should be applied in accordance with the manufacturer’s instructions and should not be commenced until after the surface defect inspections of the anode have been completed.

Anode Insert Manufacture

Insert materials should conform with the requirements of NACE standard SP0387. Inserts for welding to the protected structure should also be traceable to a certificate according to BS EN 10204 or ISO 10474. Individual anodes should also be traceable to the corresponding welding certificates.

All fabrication welding of anode inserts shall conform with the requirements of NACE standard SP0387 and should include a visual inspection just before the casting of the anode. This should conform with the manufacturer’s Manufacturing Procedure Specification (MPS).

Anode Marking and Documentation

Each anode produced should be marked with the manufacturer’s name or symbol, the anode material, heat number and serial number. Further requirements may be requested by the purchaser.

Anode Handling, Storage and Transportation

Anodes should be handled, stored and transported in accordance with NACE standard SP0387. Any additional requirements for the specific anodes should be specified by the purchaser.

Anode Inspections and Testing - Pre-Production Qualification Testing (PQT)

The purpose of the PQT is to verify that the Manufacturing Procedure Specification (MPS) is adequate to achieve the required anode performance properties. This is particularly important for performance properties which require verification with destructive testing which cannot therefore be verified frequently during the manufacturing process. A PQT should be produced by the supplier for any anode orders greater than around 15,000kg, though the purchaser may request one is done for smaller orders.

Special requirements of the PQT such as number of anodes to be inspected and number to be destructively tested should be specified in the purchasing documents so the supplier can allow for the required tests to be undertaken. The instruments required and any special requirements for calibration and certification should also be specified in the purchase documents.

The supplier will produce a PQT report detailing all the results from the required tests and inspections. This will be delivered to the purchaser either at the end of the manufacturing process or interim reports can be compiled and issued during the manufacturing process if required by the purchaser.

Anode Inspection and Testing Plan (ITP)

The supplier will also supply an ITP for all orders greater than around 15,000kg. The ITP should detail all the quality control procedures, tests and inspections which will be undertaken during the manufacturing procedure. Each activity should be listed in consecutive order and should include reference to the applicable codes and standards as well as the supplier’s procedures. The ITP should include details of the instruments used, the frequency and extents of all testing and inspection activities, the acceptance criteria used and the procedures for dealing with an non-conformances identified as part of the process.

The purchaser should have the right to inspect any aspect of the manufacturing process as required and can carry out an audit of the supplier’s procedures if necessary. The purchaser should also agree any hold points required within the manufacturing process to allow the necessary inspections to take place.

A typical ITP may include items for the following;

  • Document approvals
  • Material receipts for anodic materials
  • Material receipts for insert materials
  • Insert shot blasting
  • Insert visual inspections
  • Insert weight controls
  • Mould inspections
  • Insert placement inspections
  • Anode weight records
  • Anode markings inspection
  • Anode samples for chemical analysis
  • Anode surface finish, appearance and quality inspections
  • Anode cracking inspection
  • Anode dimensional checks
  • Destructive testing records
  • Anode coating inspections
  • Electrochemical testing records
  • Packing inspections
  • Review and documentation checks

Anode Electrochemical Testing

Anode electrochemical testing is important as it shows that the anodes will perform as expected in the design and should generally be required for orders greater than around 15,000kg. Testing should be carried out as part of the PQT or on the first day of manufacture and should be repeated for each 15,000kg of anodes produced.

Sampling for testing should be carried out for each heat produced or for aluminium anodes two samples for all heats exceeding 500kg. The samples should be taken at the start and/or the end of the pouring stream. The testing should be undertaken in accordance with DNVGL-RP-B401.

The test procedure should be selected to provide the best representation of the expected operating conditions. This should include for variations in the electrolyte, operating temperatures and the anode current density. For large scale cathodic protection projects subject to unusual operating conditions or where a new alloy is proposed, field testing may be justifiable. This field testing should be undertaken on a large enough scale to produce statistically meaningful results. Field tests should be conducted or witnessed by an independent inspection authority.

There are three main methods of testing;

  • Free Running Test – The anode is connected to a steel cathode and the polarization behavior is produced by the galvanostatic effect under given conditions. The current and anode potential can be measured during the test and afterwards the anode electrochemical capacity is measured. An example procedure for a free running test is provided in DNVGL-RP-B401. This test should run for a minimum of 12 months. Long term testing is the only way to accurately predict electrochemical capacities and potentials for design purposes.
  • Galvanostatic Test – The anode is exposed to a defined constant current density. The anode potential is measured during the test and afterwards the anodes electrochemical capacity can be measured. Note the constant current density does not mimic real life conditions so passivation effects cannot be assessed. An example procedure for a galvanostatic test is provided in NACE TM0190-98. Short term tests such as this do not produce reliable design info as they do not simulate real life situations. This test is only useful as a means of quickly checking the quality of newly produced anodes where design information is known.
  • Potentiostatic Test – The anode is exposed to a defined potential. In practical situations anodes will operate with a relatively stable potential, while the current will vary significantly. However it is difficult to estimate the operating potential and if this is incorrect the current output will be unrealistic. Therefore this test is only useful where detailed knowledge exists on the operating potential in the specific environmental conditions. Sometimes tests are undertaken over a range of potentials. An example procedure for a potentiostatic test is provided in ASTM G5-94.

The short term galvanostatic testing acceptance criteria for anodes using the conservative design values shown elsewhere in this document should be as follows;

  • Aluminium anodes should have an electrochemical capacity greater than 2,500 Ah/kg and a closed circuit potential more negative than -1.05V at the end of the 4th testing period.
  • Zinc anodes should have an electrochemical capacity greater than 780 Ah/kg and a closed circuit potential more negative than -1.00V at the end of the 4th testing period.

For anodes designed using higher electrochemical capacities or potentials may require higher acceptance values.

The dissolution morphology of the anode should also be noted following testing as this can give an indication of whether active or passive behavior can be expected. It will also indicate whether there is a potential for undercutting. Undercut corrosion pits can develop due to an acidic environment which can form in the pit due to restricted electrolyte access. This can lead to accelerated self-corrosion within the pit and corresponding reduced anode design life. Undercutting pits can also destabilize the support from the anode insert leading to potential losses of material and reduced utilization factor and design life.

An inspection should also be done to check for the presence of inter-granular or inter-crystalline corrosion. This type of corrosion can lead to a rapid disintegration of the anodic material and therefore greatly reduced anode life.

Chemical Analysis

Sampling for chemical analysis should be undertaken in accordance with NACE standard SP0387. Samples should be taken for each anode heat. For aluminium anodes two samples should be taken for all heats exceeding 500kg. The samples should be taken at the start and/or the end of the pouring stream.

For spectrometric analyses of anode chemical composition, reference standards with a known chemical composition verified by an independent party should be used.

Anode Mass and Dimensions

Verification of the anodes mass and dimensions should be undertaken in accordance with the frequency and acceptance criteria specified in NACE standard SP0387. The purchaser may request additional testing if required. Generally around 10% of the total number of anodes should be checked.

The acceptance criteria should be as follows for stand-off and flush mounted anodes;

  • Anodes with a net mass greater than 50kg shall be have a mass of ±3% of the nominal net mass
  • Anodes with a net mass less than 50kg shall be have a mass of ±5% of the nominal net mass
  • Anode mean length should be ±3% of nominal length or ±25mm, whichever is smaller
  • Anode mean width should be ±5% of nominal mean width
  • Anode depth should be ±10% of nominal mean depth
  • The diameter of cylindrical anodes should be ±2.5% of nominal diameter
  • The anode straightness should not deviate more than 2% of the anode nominal length from the longitudinal axis

The acceptance criteria should be as follows for stand-off and flush mounted anodes;

  • Anodes with a net mass greater than 50kg shall be have a mass of ±3% of the nominal net mass
  • Anodes with a net mass less than 50kg shall be have a mass of ±5% of the nominal net mass
  • Anode mean length should be ±3% of nominal length or ±25mm, whichever is smaller
  • Anode internal diameter should be;
    • -0/+4mm for diameters < 300mm
    • -0/+6mm for diameters < 600mm
    • -0/+1% for diameters >610mm
  • Anode thickness should be ±3mm of nominal thickness

Surface Inspections

The surface inspections for cracks and other defects should be carried out on all anodes with the acceptance criteria as specified in NACE standard SP0387. Note that cracking occurs naturally due to the different coefficients of expansion of the anodic material and the insert material. These small cracks are not detrimental in themselves, so long as they cannot destabilize the anodic material from the insert.

  • No cracks visible to the naked eye are acceptable for zinc or magnesium anodes
  • No cracks which penetrate to the anode inserts are acceptable
  • No cracks which penetrate all the way through an anode
  • No cracks in the longitudinal direction for long slender anodes
  • No visible cracks in the area of anode which is not supported by the anode insert
  • Within the section of anode fully supported by the anode inserts, cracks of width greater than 2mm can only be accepted when the length is less than 100mm.
  • Maximum 10 cracks per anode, small dense cracks can be considered one crack and cracks less than 0.5mm need not be included in this count
  • Shrinkage depressions should not exceed 10% of the nominal anode depth, as measured from the uppermost corner to the bottom of the depression.
  • Shrinkage depressions must not expose the anode insert.
  • Cold shuts should not exceed a depth of 10mm or extend over a total length of more than 3 times the anode width.
  • There should be no protrusions which may be hazardous to personnel handling the anodes.

Anode Inserts Inspections

The condition of the anode inserts should be inspected just prior to the casting of each anode. The insert must be checked for any signs of rust discoloration or surface contamination present.

The positions of protruding anode inserts should comply with the tolerances stated by the manufacturer. This should be checked for a minimum of 10% of the anodes of each design. This may be extended by the purchaser if required.

A minimum of two anodes of each size should be subject to destructive tests to verify the location of anode inserts.

Coatings

Any coatings applied shall be visually inspected on all anodes concerned. Any spills of coating materials on anode surfaces which are not to be coated shall be removed.

Destructive Testing

A minimum of two anodes of each size shall be subject to destructive testing. This testing should check for the presence of any internal defects as well as the position of the anode inserts. The cutting procedure and acceptance criteria should be in accordance with NACE standard SP0387.

Sections should be cut at 25%, 50% and 75% of the anode length. The following acceptance criteria should be met;

  • Visible gas holes and porosities should be less than 2% of the total section surface areas, and less than 5% of any individual section surface area.
  • No cavity should exceed 1cm2 in cross sectional area.
  • Non-Metallic inclusions should be less than 1% of the total section surface areas, and less than 2% of any individual section surface area.
  • No non-metal inclusion should exceed 1cm2 in cross sectional area.
  • Voids adjacent to the insert indicating a lack of bond should not exceed 10% of the total anode insert perimeters, and less than 20% of any individual insert perimeter.

The above limits may be relaxed for non-tubular anodes where prevention of voids may be difficult if agreed with the purchaser.

The acceptance criteria for steel inserts should be ±5mm for bracelet and flush mounted anodes, and ±10% of the anode thickness covering the insert for stand-off type anodes.

This testing should be carried out as part of the PQT or on the first day of manufacture. The purchaser shall have the right to select which anodes are tested.

Non-Conformances (Other than Electrochemical Properties)

Where non-conformances are identified during testing (other than for electrochemical properties), the preceding and following anodes should be tested until 3 successive anodes are acceptable. Where repeated failures are identified production should be discontinued and the supplier should issue a non-conformance report. This should include details of the cause of the failure and measures taken to rectify it.

Records

All records of testing and inspections should be documented and should be traceable to a unique anode or batch number. An inspection document should be issued by the manufacturer according to BS EN 10204 or ISO 10474.

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