Corrosion typical pattern of worm-like threads or filaments is present . It is not possible to precisely rate FFC , so ISO calls for an assessment of filament length and frequency ( ISO 4628-10 ) and suggests that photographs may be helpful in determining the extent of FFC .
ASTM D2803 The standard guide calls for coated metal specimens to be scribed and placed in a corrosive atmosphere to initiate ( filiform ) corrosion . The specimens should then be exposed to controlled temperature and RH conditions “ known to be conducive to filiform corrosion ”. ASTM D2803 states that FFC usually occurs between 20-35 ° C ( 68-95 ° F ) with a corresponding RH between 60-95 %. The main differences between ASTM D2803 and ISO 4623-2 is that the ASTM standard guide uses salt spray ( as per ASTM B117 ) to initiate corrosion as well as different temperature and RH ranges and specified test durations .
HVAC / R coil coatings HVAC / R assets can be protected from highly corrosive and perhaps chemically aggressive service environments through the application of a high performance HVAC / R coating to the coil unit . When considering the range of HVAC / R coatings on the market , although no specific coatings will be referenced in this article , it seems likely that chemical-cured or baked coatings by virtue of their densely cross-linked structures would exhibit a significantly lower WVTR compared to quick- ( air ) -dry coatings , and thus be more robust with respect to moisture permeation and hence resistant to FFC . It could also be anticipated that these chemically crosslinked products would afford superior “ wet ” adhesion compared to those air-drying materials , thus rendering them less susceptible to deformation or delamination in the event of FFC being initiated . There is a growing trend in the industry to balance absolute performance from a coating solution with a solution that provides a more environmentally friendly impact on our world and is inherently safer to apply by coating technicians ( and dispose of ) due to the waterbased chemistry of the formulation . Water-based coatings contain fewer or zero VOCs ( volatile organic compound ) and other hazardous chemicals . Their application is not restricted by international EHS guidelines , although normal PPE should be worn . Formulation know-how in the 21 st Century has also allowed water-based ( acrylic ) coatings to be created with enhanced properties through the incorporation of special additives . One such additive can generate self-healing properties , so if a small area becomes compromised , the coating ' s selfhealing properties can activate and “ repair ” the damage . The additive reacts with moisture to form a protective layer over the site , thus helping to prevent the spread of corrosion . Advanced corrosion inhibitors can also be added as well as de-foaming agents to functionalise and optimise corrosion protection and application performance respectively . Water-based ( acrylic ) coil coatings as with all protective coatings do require some coating application know-how and are easy to apply using a variety of methods , including spray , flow coat ( flooding ) application , and even roller . This allows easy set-up in an OEM plant , at a coil coating applicator , or at site without major EHS issues .
HVAC / R coating type and enhanced surface preparation Whilst the selection of specific coating type may play a significant role in minimising the incidence of filiform corrosion , surface preparation could be argued is equally important . Thorough cleaning of the substrate , such as removal of soluble salts and any ( stamping ) oils , grease etc ., should be considered an absolute minimum requirement . However , given the geometric complexity of aluminium-finned heat-exchanger units , an appropriate chemical pre-treatment or conversion process , prior to application of the coating , should also perhaps be given serious consideration to provide additional insurance against FFC , particularly where warm , humid , salt-laden service environments may be anticipated . A silane pre-treatment can provide added protection against FFC . It works by creating a chemical bond between the aluminium substrate and the water-based ( acrylic ) coating , which enhances the coating ' s adhesion and prevents water from penetrating the coating and reaching the aluminium surface .
Conclusion Filiform corrosion does exist , and aluminium HVAC / R fins are not immune from attack , when protected by an organic coating . What is critical is that the basics are understood – when it occurs , what it looks like , how it forms / initiates , and the results once it has propagated . It can be identified . Its appearance is unique ( fine filaments ). And it can be tested for using ISO or ASTM standards . During the design process for the coil , it is fundamental that the right protective coating is specified . The fins may be susceptible to filiform corrosion when a high performance coating is applied to the aluminium substrate in conditions of high salt concentration , elevated temperature and raised humidity . This can result in the creation of unsightly white corrosion product , and this surface effect will affect HVAC coil operational efficiency . It is possible to mitigate the risk of occurrence of filiform corrosion through the selection of a HVAC / R coating system that offers superior wet adhesion with and barrier properties to prevent the permeation of moisture to the substrate . Correct and thorough surface preparation is also vital . Nowadays , one popular choice for this application is a waterbased acrylic coil coating , where reliable , durable protection is sought coupled with strong eco and EHS credentials .
References
1 . VARGEL , C ., “ Corrosion of Aluminium ”, Elsevier , 2004 . 2 . MCMURRAY , H . N . and WILLIAMS , G ., “ Under Film / Coating Corrosion ”, Reference Module in Materials Science and Materials Engineering , Elsevier , 2016 .
3 . YASAKAU , K . A ., ZHELUDKEVIC , M . L . and FERREIRA , M . G . S ., “ Role of intermetallics in corrosion of aluminium alloys . Smart corrosion protection ” in Intermetallic Matrix Composites : Properties and Applications , Woodhead Publishing , an imprint of Elsevier , 2018 .
4 . “ Corrosion Basics ”, https :// www . ampp . org / technicalresearch / impact / corrosion-basics , AMPP , Accessed : 17 / 02 / 2023 .
About the author
Craig set up his own international consultancy business in 2016 , focusing most recently on HVAC / R coatings . He has over 20 years ’ experience in the field of protective coatings , and has worked with / for 3M , Copon-E Wood , Jotun , Kansai Paint , and Heresite . He is a certified NACE Coating Inspector ( Level 1 ) and an ASHRAE Associate Member . Craig is also a Brand Ambassador for JettyRobot – supplier of unique and patented robotic HVAC duct and pipe cleaning technology . www . heat-exchanger-world . com Heat Exchanger World April 2023
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