Several requests for proposals have recently been published related to Al-Zn-In anodes that are to be applied in cathodic protection (CP) systems as galvanic anodes. For example, we can consider a very recent construction publication by the Secretary of Urban Development, Public Works, and Infrastructure from Campeche State in Mexico for the acquisition of galvanic anodes of Al for the piles of the new bridge La Unidad, which specifies:
“The anode supplier must be certified with a Quality Management Certification such as ISO 9001 to manufacture sacrificial anodes with the technology of fusion and agitation by electromagnetic induction.”
Another example is the technical specification prepared by PetroEcuador, “Galvanic Aluminum Anodes” for CP of the piles of the Monteverde Marine Terminal, which states:
“12—Requirements for the Anode Fabricator
“The anode fabrication should be done using electromagnetic induction furnaces to reduce nonmetallic impurities in the alloy.
“Anode production on direct flame furnaces will not be accepted.”
Then, the question is whether this kind of anode should be manufactured with this particular process. We need to remember that the foundry furnaces can use different types of heating using electricity, gas, or fuel oil.
The sacrificial anodes’ performance is fundamental since any problem causing the anode not to perform correctly can create corrosion complications that could cause critical maintenance problems, even to the point of structural collapse. The above can lead to worse problems, such as severe production losses, environmental risks, and most importantly, risk of lives.
That is why it is of vital importance that a reasonable control exists for the fabrication of the aluminum anodes. For example, in regard to their chemical composition, since the alloy impurities can cause partial inactivity on the anode, even a change in polarity, and could even get to cases where the anode gets passivated as we will see further on, the main standards and best practice codes, national and international, have been revised to prove which parameters can determine and ensure the quality of the obtained anode and come up with the furnace selected to be manufactured.
Foundry Process on Aluminum Anodes
A brief description of the indium-activated aluminum anodes’ fabrication process and metallurgy can benefit the reader. Indium-activated Al-Zn-In is generally cast in open permanent molds.1 The molding (casting) process is an art learned throughout the experience, as Scantlebury1 mentions. The casting parameters are generally selected to avoid macrostructural defects (such as cracks, cavities, etc.), affecting the anodes’ performance.2 The NACE SP03873 and Australian Standard AS22394 provide the guidelines that drive the metallurgic requirements that must be accomplished by these melted galvanic anodes used for applications in the marine environment.
The metallurgic properties of the AlZn-In anodes are influenced by:
1. The type and grain sizes.
2. Interdendritic spacing.
3. Their nature, type, and distribution of alloying elements.
All these factors are interrelated and depend mainly on the solidification rate of the anodes. The initial crystallization in indium-activated aluminum anodes starts with aluminum dendrites, surrounded by a continuous eutectic network. The solidification of these anodes is usually by heterogeneous nucleation, and the first crystals to form generally are located at highly favored nucleation sites such as the mold walls.
As the solidification border progresses inward toward the mold’s center, the solute is pushed from the solid into the liquid phase. Additional crystals begin to form at less favored nucleation sites, such as impurities in the melt. These two processes create a macrostructure containing a fine-grained chill zone next to the mold walls, an intermediate columnar grained area, and a central, large-grained equiaxed site. The microstructure in each region naturally contains primary aluminum crystals or dendrites, surrounded by closely continuous networks of complex eutectic solids, second-phase particles, intermetallic compounds, and inclusions.
The overall solidification rate can significantly affect both the macro- and microstructural characteristics previously described. In general, faster solidification rates provide a thicker chill zone relative to the thickness of both the columnar and equiaxed zones. The fastest solidification also provides smaller grains than the bigger grains formed on a slower solidification process.
The distribution of the alloying elements, either as a solid solution, second-phase segregates, intermetallic compounds, or inclusions, affect the behavior of the ternary aluminum anodes (Al-Zn-In). Pure aluminum owns the critical characteristics2 needed for sacrificial anode properties, such as light weight and low cost. However, pure aluminum as anode material in seawater is not convenient due to the formation of a protective oxide film that limits both its current output and its potential.
Therefore, Al usually is alloyed with other elements to promote depassivation (the oxide film’s breakdown) and shift the metal’s operative potential to a more negative value. The alloying elements used to achieve this are denoted as depassivators and modifiers.
An induction furnace is an electric furnace where the heat is created by the electric induction of a conductive medium (a metal) on a crucible. Some magnetic coils are wound around them.
The specification showed at the beginning indicating that these types of ovens minimize the number of nonmetallic impurities on the alloy is quite debatable, and support has not been found on the same scientific literature consulted. Johnson,2,5 one of the experts on this subject, indicates the following:
“Those using induction furnaces for melting aluminum to make anodes are trying to imply better alloy mixing with induction over gas-fired furnaces. The induction process automatically mixes the alloy, so no stirring is needed, as required in gas-fired alloying. In an induction furnace, the molten aluminum is continuously rolling in (actually rolling out on itself). So when you drop alloy elements in the molten high purity aluminum, it gets mixed in due to the induction process. In gas, you have to mix the alloy to get a homogenous alloy physically. However, induction makes a lot of oxide during the process of the molten aluminum rolling in on itself as the process keeps breaking the surface of the molten aluminum making much more dross that needs to be removed before pouring.
“All this is to say it does not matter which way you melt it as the finished alloy can both be mixed and alloyed the same. The induction melted alloy may have more dross inclusions since that process generates a lot of it over gas-fired melting. What matters most is the alloy’s spectrographic analysis before casting, as that verifies the alloy is appropriately mixed and made to specification. The mixing process between the two melting processes is inconsequential as the analysis is what matters.”
The different manufacturers of aluminum anodes are driven by different standards, such as each country’s, British, European, or international standards. Still, all of them produce the anodes according to these standards. They also perform other types of tests, such as chemical certificates from testing laboratories or accelerated corrosion tests according to NACE TM0190-20176 or DNVGL-RP-B401.7
For the specific case of this study, a cautious review of these standards was made:
• API RP 651.10
• BS 7361.11
• BS EN 12496.12 This European standard specifies the minimum requirements and gives recommendations for the chemical composition, the electrochemical properties, the physical tolerances, and the test and inspection procedures for cast galvanic anodes of aluminum, magnesium, and zinc-based alloys for CP in seawater and saline mud. This standard applies to the majority of galvanic anodes used for seawater and saline mud applications (i.e., cast anodes of trapezoidal, “D,” or circular cross-section and bracelet-type anodes. The general requirements and recommendations of this standard may also be applied to other anode shapes (e.g., half-spherical, button, etc., which are sometimes used for seawater applications).
• NACE SP0387-2019.13 This standard defines minimum physical quality and inspection standards for cast sacrificial anodes for offshore applications.
• NACE SP0176-2007-SG.14 This standard lists materials, practices, and methods of corrosion control for fixed offshore structures associated with petroleum production. Three major areas are the submerged zone, the splash zone, and the atmospheric zone.
• NACE SP0169-2013.15
• NACE SP0492-2016-SG.16 This standard sets minimum physical quality and inspection standards for cast sacrificial anodes for offshore pipeline applications. The standard applies to typical half-shell or segmented bracelet-type anodes and is not intended to apply to the platform, hull, tank, or extruded-type anodes. The section on physical requirements includes information on samples for chemical analysis; anode identification , weight, dimensions, and straightness; insert dimensions and position; insert material; fabrication of inserts by welding; insert surface preparation; surface irregularities on the anode casting; cracks in cast anodic materials; defects; and more.
• NACE TM0190-2017.17 This standard determines the potential and current capacity characteristics under laboratory conditions for aluminum and zinc alloy anodes used for CP.
• NACE SP0196-2015.18
• NACE SP0492-2006.19
• NORSOK M-50322
• ISO 15589.23 This specifies requirements and gives recommendations for the pre-installation surveys, design, materials, equipment, fabrication, installation, commissioning, operation, inspection, and maintenance of CP systems for offshore pipelines for the petroleum, petrochemical, and natural gas industries as defined in ISO 13623. It applies to carbon steel, stainless steel, and flexible pipelines in offshore service and retrofits, modifications, and repairs made to existing pipeline systems. ISO 15589 is applicable to all types of seawater and seabed environments encountered in submerged conditions and risers up to mean water level.
• MIL-DTL-24779C (SH)24
• AS 223925
• ASTM E1251-17a26
None of the above standards include information on the type of furnace that should be used to obtain the anodes with the quality and specifications indicated in each of these standards.
From the review that was carried out of the specifications, codes of recommended best practices, and standards within our reach: ASTM, DNV, NORSOK, ISO, NACE, AS, and NRF PEMEX, it was not possible to find any reference to the manufacturing process of Al anodes, either through the use of gas or induction furnaces. This includes the particularity of using the technology of fusion and stirring by electromagnetic induction.
The anode’s quality is judged by its traditional chemical and traditional electrochemical properties: chemical composition, operating potential, electrochemical efficiency, current drainage capacity, and microstructure (which may be considered in some cases), not specifying the fusion technique used to qualify the quality of the anode. The functionality of the anode obtained is judged, not the manufacturing process applied to bring this functionality.
References and About the Author