Stop the Madness! Corrosion Causes 40% of Equipment Failures
Updated: Jan 13
THE CASE FOR IMPROVING CORROSION CONTROL
Corrosion alone accounts for approximately 40% of all equipment failures in vulnerable HVAC equipment and heat exhangers.
The link between high ROI and improved metal hygiene, versus simple surface prep, is obvious. Stronger, more comprehensive coating adhesion results in fewer coating maintenance events and by extension, lower coating maintenance events and improved protection from filiform, or under film, corrosion. By mitigating the threat of corrosion under coatings, more ROI can be derived from each HVAC and heat exchanger. Filiform, or under film corrosion is a type of "localized" corrosion and is normally linked to magnesium and aluminum alloys that utilize an organic form of coating. However, it can also occur on other coated metals such as steel, iron and zinc. Nitrates, sulfates, carbonates and condensates that contain halides have been associated with filiform corrosion. In places where the corrosion has taken place, there is a thread-like filament appearance that forms under the coating. The coating will bulge and have an appearance like that of a lawn riddled by the tunnels of a mole, due to the effect of the corrosion. The filament will then continue to form up to the points where the coating is no longer continuous.
WHERE DOES CORROSION BEGIN Contaminants in an environment typically result in the creation of electrolytes that trigger the corrosion process, in combination with other factors such as humidity, water, fog, temperature, proximity to pollutant source, impurities at the metal surface, and dust or particle contamination.Urban and industrial dust particles laden with harmful metal oxides, chlorides, sulfates, sulfuric acid, carbon, and carbon compounds in the presence of oxygen, water, or high humidity environments are highly corrosive. Premature corrosion of heat exchangers are an even more serious problem in seaside and industrial environments (where coils may fail in less than a year), exacerbated by the presence of flue gas, sewage vents or open sewage systems and diesel exhaust which add to the detrimental effect. ROLE OF SURFACE PREP Coating is a protective technique whereby the coating forms the barrier between the metal material and ambient conditions. As such, coating performance is directly related to coating adhesion. Residual underfilm contaminants negate or prevent comprehensive, strong adhesion which is especially important in areas where the coating must be thin and flexible.Wherever damaged or delaminated coating exists, corrosion will form and begin to corrode the underlying metal. While applying and maintaining uniform, comprehensively bonded and intact coatings is critical to coil service life; surface cleanliness at a high level of surface hygiene is critical to coating service life. Thus, the first step in corrosion control is surface preparation. Seems simple, right? But removing all contaminants from a surface (without introducing others to the surface) can be tricky. ACHIEVING OPTIMAL METAL HYGIENE Visible contaminants are generally removed by mechanical means through abrasive blast, hydrojetting or some combination of the two. In above-freezing temperatures, wet abrasive vapor blasting achieves the best result for several reasons. First, the process relies on force derived by increasing the mass of grit media by encapsulating it in vapor droplets. This allows considerable force to be delivered on impact using low pressure/velocity which significantly reduces the risk of embedding impurities, gouging metal from uncushioned grit, creating heat from friction, or static buildup that can draw dust to contaminate the surface, unlike high pressure methods such as dry abrasive blast or hydrojetting that rely on high pressure/velocity alone (brute force) to create the necessary impact. Such brute force causes unintended damage, drives water, grit and/or contaminants into surfaces, endangers workers, and wastes immense amounts of media and/or water. Though mechanical methods do an adequate job of removing oils, dirt, grime, coatings, rust and other visible contaminants, they are not designed to deal with soluble salts and other invisible contaminants, such as strongly-bonded iron sulfides and chlorides that are essentially a ticking time bomb for spontaneous and unexpected corrosion under coating that occurs due to osmosis and capillary moisture transfer under coatings.
“Iron sulfide is insoluble therefore water cleaning is not possible. Sulfides penetrate into the intergranular crevices in metal substrate” Vincent (1998).).
These sulfide compounds tendency to aggregate in metal surfaces during fabrication. During transportation, storage, installation and in service, these contaminants draw soluble salts through ionic attraction, even when sulfur is present at very low ppm. Soluble salts form a very strong bond with the adverse anions, rendering them ‘functionally insoluble’ (not readily removed by neither hydrojetting, abrasive blasting or salt removers).
"Soluble salts often contain chloride, nitrate, and sulfate as adverse anions. Although most of the salts are soluble in water, they cannot be easily removed from steel surface by washing or abrasive blasting. Salts may also exist in pits and crevices on corroded surfaces within or under rust." Virmani (January 2014).
These hygroscopic microcontaminants, although soluble, are thus not readily removed by washing or abrasive blasting alone.In addition, the strong attraction tends to ‘hide’ the salts below the oxide film that develops, making them virtually impossible to detect, quantify and qualify by conventional means, including conductivity testing, as confirmed by Scanning Electron Microscope & Energy Dispersive X-Ray Spectrometer Analysis. A novel decontamination chemical treatment, OxNot CleanBlast eradicates both soluble and functionally insoluble ionic and highly hygroscopic microcontaminants (i.e. sulfides, sulfates chlorides, nitrates and microbial by-products) from metal surfaces by penetrating the sulfide film, removing the ionic attraction, and rinsing away microcontaminant detritus when added to the water tank for application during regular abrasive vapor blast. Unlike nearly all rust removers, inhibitors and salt removers, this novel treatment leaves no traces of film-formers or compounds that interfere with coating adhesion. OxNot leaves nothing but clean metal behind and does not affect intact coatings (even galvanized), allowing unimpeded and comprehensive coating bonding. Tests confirm that metal decontamination using OxNot significantly improves adhesion on even unprofiled metal.
THE BOTTOM LINE In the past, corrosion was accepted as inevitable. Achieving optimal metal hygiene (the key to heightened corrosion control) today is not only a real possibility, but smart business. Due to the development of vapor blast and new chemical treatment technologies, switching the focus on metal hygiene, rather than simple surface preparation, promises to be a real game changer with potentially great financial advantages for industries operating heat exchangers and HVAC equipment.
1. Vincent, L.D. “Decontamination of Metal Substrates”. CORROSION 98, paper no. 98620. Houston, Texas: NACE International. Houston TX. March 22-27, 1998. p 4.
2. Virmani, Paul Y. “Methodology for Analysis of Soluble Salts from Steel Substrates”. FHWA Long-Term Bridge Performance Program Summary Report Federal Highway Administration. HRT-14-026..January 2014. p 1.