Recognising “hidden” formicary (ant nest) corrosion

Formicary, [1] or ant-nest corrosion [2] – based upon the similarity of substrate damage with an ant’s nest (Figure 1) – dates back about fifty years. [3,4]. Formicary corrosion is a rapid acting process involving the conjoint presence of oxygen (air), moisture and a weak organic acid. It may be regarded as a “hidden” phenomenon because, (a) it is unknown to those who have not experienced it first-hand, and (b) it is challenging to identify, because the surface pin hole leak sites are often so fine that they are not visible to the unaided eye (Figure 2). Generally considered unique to copper, this insidious form of internal corrosion (Figure 1) comprises of micro-pitting networks (akin to tunnels) [5,6] that can fully penetrate small bore tubing in weeks, not years. Pin holes that occur on the outer or inner diameter surfaces are commonly surrounded by zones of discoloured copper, ranging from dull gray/black to red/brown or purple hues (Figure 2). Refrigeration-grade copper tubing, [7] used throughout the HVAC industry, was ravaged for decades (Figure 3) and 10% of all premature failures were attributed to formicary corrosion.[8]

Figure 3: Formicary corrosion common to all HVAC coil manufacturers.

red/brown/purple hues on copper surface.
Figure 2: Tiny pin hole leak location.

Photomicrographs from different manufacturers; depth of polishing affects the visible spread of damage shown

[9] As-polished.

Copper used in other applications, including heat pumps, dehumidifiers, air coolers, heat exchangers, freezers, and chiller units, can experience formicary corrosion. Identical penetrating attack observed under coatings, lagging, sealant contact, and insulated copper piping, is typically the result of wet conditions and compounds that hydolyse to form carboxylic acids, or other chemical sources, including chlorinated organic compounds (trichlorethane, trichloethylene, etc.), and hydrolysis products from the decomposition of esters, aldehydes and alcohols (carbonyls).

Mechanism of formicary corrosion
Extensive research by the Japanese Copper Development Association [10,11] showed that formicary corrosion occurs when certain residual organic compounds degrade in the simultaneous presence of air and moisture, to produce carboxylic acids, such as formic, acetic, propionic and buteric acids. This finding was pertinent to the HVAC industry (Figure3), where synthetic lubricating oils used for forming coils, and degreasers/detergent cleaners, contained such compounds.
The general mechanism involves a micro anode, where dissolved copper ions combine with carboxylic acids (HCOOO-) to form an unstable cuprous complex Cu(CHOO), which is oxidised to form cupric formate, acetate, etc., 2Cu(CHOO)2 (cuprous complex) and cuprous oxide (Cu2O).
Microcracks with localised intergranular attack, caused by a wedging effect from the volume expansion in forming the cuprous complex and cuprous oxide [6,12] initiate at weaknesses along the corrosion pit wall, exposing more surfaces of copper to perpetuate the advancing corrosion process.

Recognising formicary corrosion
Once the locations of the fine pinholes are found – by water immersion pressure testing, macroscopy, or both – optical microscopy will confirm if the images display the unique metallography of the formicary tunnels (Figures 1, 3), which are significantly different from the smooth hemispherical contours associated with general pitting corrosion of copper (Figure 4).

General pitting by halides (chlorides, fluorides). Comparison from coil manufacturer’s report[9]. Subsurface tunnels of formicary corrosion.
Figure 4: Contrasting the two most cited common forms of coil corrosion.

As industries have become more familiar with the causes of formicary corrosion, the frequency of leaks has fallen. Research [13]1 sponsored by the Air Refrigeration Technology Institute (ARTI), to develop a reproducible screening method to determine the mechanisms and effects of corrosion,[14] resulted in a hydrolytic stability test, which uses ion chromatography to ensure that lubricant drawing oils and finning lubricants are free of carboxlic species. A separate pre-screening hexane rinse of copper tubing, provides evidence of residues with carboxylic species identified using Fourier Transform Infrared Spectroscopy (FTIR). Several HVAC equipment manufacturers have made material and design changes, including, tin-plated copper hair pins and tubing less prone to attack, ductless AC units with moisture filters, and all-aluminium tubing and fin construction.

Currently, indoor coil leak failures by formicary corrosion have been attributed to energy- efficient buildings with decreased ventilation that promulgate higher concentration levels of carboxylic acids from building materials, woods, adhesives, disinfectants, cleaning solvents, vinegar seasonings, liquid smoke, cosmetics, etc.

Selected examples of formicary corrosion leakages presented herein, include, HVAC coils with pin holes on the outer (OD) or inner (ID) diameter surfaces (Figures 5, 6), and a heat exchanger from a fabrication shop (Figure 7).

OD-initiated formicary intrusions in defect-free copper.

ID of split tube with purple/red-brown surface film. pin hole in purple/red-brown surface film.

ID-initiated formicary intrusions in defect-free copper.

Open end of heat exchanger. Pin hole in purple/red-brown surface film on ID of tube.

ID-initiated coarse formicary tunnel in defect-free copper.

Pin holes in copper are not always formicary corrosion
Some examples of formicary corrosion have been presented in the previous paragraphs, with brief comments about how it can be assessed and generally avoided by using screening tests on the copper tubing and of the contact environment. Presented In the following paragraphs are some examples where pin holes were wrongly ascribed to formicary corrosion.

• Failed HVAC installation – pin holes found following about 2 to 3 years service were claimed by the coil supplier[15] to display the “worst case of formicary corrosion ever seen”. They were incorrect. The observed damage (Figure 8), and the optical photomicrographs (Figure 9), show no micro-pitting networks – the chacteristic feature for formicary corrosion.

The leak locations along the copper tubing appear to be associated with the design and assembly of the ceiling-mounted units. The repeating geometric pattern of hemispherical- shaped wastage zones close to, or in direct contact with, the aluminium fins, (Figure 9) suggests a modified form of Rosette corrosion,[16] combined with crevice corrosion,[17] and galvanic attack. [18] The additional pitting in the vicinity of the through-wall leaks (Figure 8) is indicative of some form of synergy between these processes.

Rosette corrosion is a somewhat recently recognised corrosion phenomenon,[19] which was encountered in copper hot water cylinders fitted with aluminium anodes. The anodes were installed to prevent type 1 pitting corrosion and was very successfully applied for over 30 years.[20] However, corrosion failures were occurring at the bottom of these cylinders where the water was cooler. This phenomenon was attributed to the interaction of the copper-aluminium galvanic couple [21] and certain impurities in the water that generated reducing species that led to the corrosion at the bottom of the cylinders. This form of corrosion has been essentially eradicated in the United Kingdom, when specifications disallowed the use of aluminium protector rods in 2002.[22] The removal of the aluminium anode and a redesign of the hot water cylinders eliminated the cold bottom thus promoting the growth of a semi-protective corrosion layer.

• Formicary corrosion by contact with sealant – accounting for a blue discoloration surrounding a usually white-colored acoustic sealant (Figure 10), a recent publication wrongly implied that tube failure by formicary corrosion and environmentally assisted cracking (EAC) from such contact was probable. [23] Long-term simulation testing with high relative humidities and temperatures showed this was most unlikely.[24] Without a sustained presence of air and moisture, through-wall penetration by formicary corrosion – a rapid process, as noted earlier – is not expected. Mechanical laps from tube-forming (Figure 10) were wrongly interpreted as EAC with formicary corrosion. This matter was addressed nine years ago, [25] the blue discoloration was nothing more than an aesthetic issue.


Discussion and conclusions
Formicary corrosion is regarded as a subsurface network of microscopic-corroded tunnels that are considerably larger than the tiny pin holes they connect to, [27] which presents a challenge to those who are aware of the “usual” localised phenomenon of pitting corrosion,which ranked fourth from seven common sources of copper tube corrosion based on over 1,500 investigations over 25 years in domestic water systems. [28] This source noted that all can be mitigated cost-effectively provided that, water quality is maintained, copper tube systems are properly designed and installed, and systems are operated within design parameters.

Formicary corrosion may be active if:
• fluids leak in weeks or months.
• surface zones of discoloured copper (dull gray/black to red/brown or purple hues) are evident.
• copper exposed to air (oxygen), moisture and a weak organic acid, e.g., carboxylic acids, (formic, acetic, propionic, buteric).
• copper exposed to other chemical sources, including chlorinated organic compounds and hydrolysis products from decomposition of esters, aldehydes and alcohols (carbonyls).
• wet conditions persist under coatings, lagging, sealant contact, and insulated piping with compouds that hydrolyse to form carboxylic
Success in dealing with formicary corrosion, like many other forms of corrosion, is dependent on knowing what it is; knowing how to recognise it; knowing what causes it; and focusing on control, rather than elimination – the realistic goal. [30]

The author kindly acknowledges the contributions of Mr. Fred Sherman, Sr. Materials Analyst, for his meticulous contributions to the metallography and laboratory testing, and Mr. Brad Krantz, VP of Laboratory Services, for support and access to Corrosion Testing Laboratories, Newark DE.

“For-mi-car-y”: from Medieval Latin, Formica ‘ant’, defined as a nest of ants or an anthill, Oxford Dictionary.
S. Yamauchi, K. Nagata, S. Sato, M. Shimono, J. Japanese Copper & Brass Research Assocn., 22, p.132, (1981).
J.O. Edwards, R.J. Hamilton, J.B. Gilmore, Materials Performance, NACE International, 16, 9, p.18, (1977).
J. M. Keyes, International Copper Research Association Symposium, Belgium, June (1965).
P. Elliott, R.A. Corbett, “Ant Nest Corrosion – Exploring the Labyrinth”, Corrosion Reviews, 19, No. 1, p.1-14, (2001).
R.A. Corbett, P. Elliott, “Digging the Tunnels”, Corrosion Reviews, 20, No. 2, p.51-66, (2002).

DHP copper, alloy C122, UNS C12200, 99.90% copper, ASTM B280.
Go Isobe et al, NACE Corrosion Asia, Paper 105, Singapore, September (1992).
Carrier Corporation, Industry Research Reports “Indoor Coil Corrosion” (2007 to 2011).
T. Notoya, T. Hamamoto, K. Kawano, Corrosion Engineering (Japan), 367, 2, p.1, (1988).
T. Notoya, “Localized “Ant Nest” Corrosion of Copper Tubing and Preventive Measures”,Materials Performance, NACE International, 32, 5, p.53, (1993).
D. M. Bastidas, I. Caynela, J.M. Bastidas, CENIM – National Centre for Metallurgical Research, CSIC, Avda, Madrid, Spain, “Ant-nest corrosion of copper tubing in air-conditioning units”, Revista de Metalurgia, 42 (5) September/October, p.367-381, (2006).
Air Refrigeration Technology Institute (ARTI), Report, 21-CR Research Project 611-50055, R. A. Corbett, Corrosion Testing Laboratory, Newark, DE, with input from P. Elliott and T. Notoya, (2003).
R.A. Corbett, “The Development of a Reproducible Screening Method to Determine the Mechanisms and Effects of Organic Acids and other Contaminants on Corrosion of Aluminium-finned copper tube Heat Exchanger Coils, Corrosion 2004, Paper 04321, NACE International,
New Orleans, LA, (2004).
15. Private communication.
Rosette corrosion – the premature failure of copper hot-water cylinders fitted with aluminium rods to prevent Type 1 copper pitting: R.J. Oliphant, Journal of Chartered Institute of Water & Environmental Management, UK, vol 14, p. 207, July (2007). R.J. Oliphant, Causes of Copper Corrosion in Plumbing Systems, Foundation for Water Research Review, FR/R0007, May (2003).
Crevice corrosion – localised corrosion of a metal or alloy surface at, or immediately adjacent to, an area shielded from full exposure to the environment because of close proximity of the metal or alloy to the surface of another material or an adjacent surface of the same metal or alloy. NACE/ASTM G193-12d, Standard Terminology & Acronyms Relating to Corrosion, (2010).

Galvanic corrosion – accelerated metal corrosion because of electrical contact with a more noble metal or nonmetallic conductor in a corrosive environment. NACE/ASTM G193-12d, Standard Terminology & Acronyms Relating to Corrosion, (2010).
R. Francis, Corrosion of Copper and its Alloys: A Practical Guide for Engineers, NACE International, (2010).
R.J. Oliphant, Journal of Chartered Institute of Water & Environmental Management, UK, vol 14,p. 207, July (2007).
21. V.F. Lacey, British Corrosion Jnl., 7, p.36, (1972).
P. Munn, Causes of Copper Corrosion in Plumbing Systems,3rdEdition, Report FR/R0007, Foundation for Water Research September, (2017).
K. Steiner, “Formicary Corrosion and EAC of Copper Tubes in Contact with Building Sealant”, AMPP Annual Conference, Paper No.17846, AMPP, San Antonio, TX, (2022).27.
P. Elliott, “Blue-Stained Sealants Do Not Imply Catastrophic Corrosion of Medical-Grade Copper Piping”, Materials Performance, NACE International, 52, 6, p.57, (2013).
25. ibid.
Laps sourced from: ASM International, Volume No. 11, Failure Analysis & Prevention, Failures Related to Metalworking, p.82, (2002); Rollason E.C., Metallurgy for Engineers, 4th Ed., Edward Arnold, p. 62, (1973).
27. A.H. Brothers, Mainstream Engineering, Rockledge, Fl – internet search.
J. R. Myers and A. Cohen, “Copper-Tube Corrosion in Domestic-Water Systems”, HPA Engineering: Supplement, June, (2005).
P. Elliott, “Fight Formicary Corrosion”, Chemical Processing, p. 37, November (2005).
Peter Elliott


Full section perforation via formicary “tunnels”. 150X Unique “ant”. 500X

Figure 1: Through-wall formicary corrosion and detail of local “ant”.

Removing aluminium fins to expose tiny holes. Soapy bubbles locate miniscule holes by immersion testing with nitrogen.
Figure 5: OD attack in HVAC coil promoted by cleaning/disinfecting wipes containing ethanol, propanol, and a propionate (a carboxylic specie).

OD-initiated formicary intrusions in defect-free copper.
ID of split tube with purple/red-brown surface film. pin hole in purple/red-brown surface film.
Figure 6: ID attack in HVAC tubing by detergent or degreaser contaminated with carboxylic specie. The tube was drawn using lube oil free from carboxylic acids.

ID-initiated formicary intrusions in defect-free copper.
Open end of heat exchanger. Pin hole in purple/red-brown surface film on ID of tube.
Figure 7: ID attack in enhanced copper tubing in a heat exchanger bundle from a fabrication shop. Surface products contained carboxylic (formate) species.
ID-initiated coarse formicary tunnel in defect-free copper.

From left to right. Bubbles show leaks. Aluminium fins stripped to access copper sample. Macro of pinholes on the OD surface.

Close up of OD pinhole leak and surrounding small pits.
Figure 8: Pin hole and other localised pits in one of many HVAC indoor coils.

Detail of OD leak location. As polished. 80X
Figure 9: Through-wall leak location coincides with a repeating geometric pattern of smooth hemispherical pits aligned with the aluminium fin locations.

Blue-stained white sealant; 18 months in service. Superficial etch stains on copper with sealant removed.

Shallow surface laps under sealant. As polished. 500x. Shallow surface laps remote from sealant. 500x

OD – longitudinal surface lap.Photomicrograph of surface laps.Schematics of OD and ID laps.

Figure 10, Blue-stained sealant on copper tubing in hospital installation and shallow surface laps [26] misinterpreted as EACs and the initiation of formicary corrosion.

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