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Corrosion
TAIL |
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H 2 O + O 2
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HEAD |
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O 2
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CORROSION PRODUCT AI ( OH ) 3
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H 2 O
O 2
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O 2
OH -
OH -
OH - OH -
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AI 3 +
CI -
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H 2 O
AI 3 +
AI 3 +
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AI 3 +
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HVAC / R COATING |
ALUMINIUM FIN |
CATHODE |
e- |
ANODE |
Figure 3 : The electrochemical process of filiform corrosion .
damaged or corroded coils are less efficient at transferring heat , which can lead to increased energy consumption and higher operating costs over time . By protecting the coils with a high-quality coating , businesses can reduce energy costs while also extending the lifespan of their equipment . Whilst it has been reported that FFC can occur with all paint and coating types 1 , field experience suggests that certain coating types are inherently less susceptible to , and others demonstrate a greater propensity for FFC to occur . According to AMPP 4 , a high level of ( wet ) adhesion allied to a low water vapour transmission rate ( WVTR ) should minimise the propensity for FFC to initiate between the coating and the aluminium substrate at a ( micro- ) defect or site of damage . Standards form the foundation of testing and allow a better understanding of coating performance . ISO and ASTM have developed two similar , but not identical tests to determine the FFC resistance of organic coatings ( on metal substrates ).
ISO 4623-2 The standard specifies a test , whereby scribe marks shall be cut through the coating applied to an aluminium panel , which are then exposed to a small amount of hydrochloric acid in the form of vapour . The panel is subsequently subjected to a temperature of 40 º C ± 2 º C ( 104 º F ± 4 º C ) and a RH of 82 % (± 5 %) in a test cabinet . The test duration is not specified . FFC is only deemed to have occurred , if the
Standard references identified in text
ASTM B117-19 - Standard Practice for Operating Salt Spray ( Fog ) Apparatus ASTM D2803-09 ( 2020 ) - Standard Guide for Testing Filiform Corrosion Resistance of Organic Coatings on Metal ISO 4623-2:2016 - Paints and varnishes - Determination of resistance to filiform corrosion . Part 2 : Aluminium substrates ISO 4628-10:2016 - Paints and varnishes — Evaluation of degradation of coatings — Designation of quantity and size of defects , and of intensity of uniform changes in appearance . Part 10 : Assessment of degree of filiform corrosion
Acronyms
AMPP – Association for Materials Protection and Performance ( formerly NACE International / SSPC ) ASTM – American Society for Testing and Materials EHS – Environment , Health , and Safety FFC – Filiform Corrosion ISO – International Standards Organisation RH – Relative Humidity WVTR – Water Vapour Transmission Rate VOC – Volatile Organic Compound
Figures 4 and 5 : Water-based acrylic HVAC / R coating applied to the coils of an air-cooled chiller ( left ) and rooftop packaged unit ( right ) in the Middle East . Courtesy of Aqua Aero Coatings BV ( URL : https :// www . aquaaero . net )
28 Heat Exchanger World April 2023 www . heat-exchanger-world . com