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Corrosion
The industry that is perhaps most affected by FFC is transportation , and in fact , it was described for the first time in 1944 , whilst being observed on coated steel and then on high-resistance aluminium ( alloys ) used in aerospace 1 . The HVAC industry and its aluminium fins are not however immune from attack from this type of corrosion .
Filiform corrosion – the basics The aim of this article is to present a simple overview of filiform corrosion , including its incidence , appearance , and formation mechanism . Some of the implications of FFC relating to thin film organic coatings will also be discussed with specific reference to HVAC / R coatings .
Incidence and appearance of filiform corrosion FFC can occur under thin film organic coatings that have a dry film thickness of less than 100 microns ( 4 mils ) on both steel and aluminium ( alloy ) structures , although it is most commonly associated with aluminium substrates . The appearance of FFC is unique ( see Figure 2 ) and bears a strong resemblance to fine filaments . These worm-like threads ( 100-500 microns ( 0.1-0.5 mm ) wide and typically 1-10 mm long ) emanate from one or more points ( coating defects or damaged coating sites ) in semi-random directions , which propagate at the metal-coating interface . The underlying metal , however , suffers only very superficial attack - generally of the order of tens of microns ( maximum ).
Mechanism of filiform corrosion FFC typically initiates “ when aggressive species , such as chloride ions , permeate through an organic coating , usually at a scratch , defect or weakness in the coating , and accumulate at the metal / coating interface ” 2 . The site of initiation tends to be an edge . Poor adhesion of a coating to an aluminium substrate or the presence of soluble salts under the coating at the time of application can also be responsible for FFC activity . Sodium chloride ( sea salt ) contains chloride ions and is classed as a soluble salt . FFC tends to be most prevalent in marine environments , although can occur in industrial locations . It can be accelerated by the presence of other soluble salts , which can enhance acidification at the corrosion site . Very simply put , a “ corrosion filament ” comprises a head and tail , which form a mobile electrochemical ( corrosion ) cell on the surface of the aluminium . The head acts as the anode , and the tail is the cathode . The electrolyte is water , and the fourth element of the cell is aluminium . At the active head , aluminium is dissolved , and inert corrosion product is created in the form of aluminium hydroxide and pushed to the tail . Aluminium hydroxide is an amorphous powder , white
Formation of a corrosion filament – The techno-chemical version
The corrosion filaments comprise a mobile , electrolyte-filled head that itself comprises metal cations ( Al 3 + ) and aggressive anions ( such as Cl - ), and a tail of dry , porous corrosion product . At the leading edge of the filament head , anodic dissolution of aluminium occurs with the formation of aluminium cations ( Al 3 + ). Transport of gaseous oxygen through the inert corrosion product of the tail continues to drive anodic metal dissolution at the leading edge of the filament head , whilst cathodic reduction of oxygen to form hydroxide anions ( OH - ) continues at the trailing edge of the head . And hence the formation of aluminium hydroxide ( Al ( OH ) 3
) occurs . Aggressive anions ( Cl - ) and water are conserved in the electrolyte in the filament head , thus enabling the corrosion filaments to propagate for lengthy periods of time ( months , or even years ) in the presence of a supply of oxygen .
in colour . If not recognised and left unchecked , corrosion filaments can propagate for months , or even years . The factors that can accelerate the FFC process are usually increases in both temperature and relative humidity ( RH ). It has been postulated that the most rapid growth of corrosion filaments occurs at a RH of about 80 %, allied to a temperature of around 40 º C ( 104 º F ) 3 . The mechanism of filiform corrosion is shown schematically in Figure 3 in its most basic form .
Implications of filiform corrosion FFC does not result in significant metal loss and is very much viewed by corrosion and coating specialists as a surface effect . However , its presence can be unsightly , and the expansive nature of the corrosion product formed result in bulging and delamination of the protective coating that has been applied . Selection of an appropriate surface preparation technique and a suitably compatible and robust material are therefore critical when considering protective coating systems for aluminium structures . In the case of HVAC / R units , where the fins are manufactured from aluminium , avoidance of filiform corrosion will be of paramount importance . The presence of corrosion product over a large area of the coil ’ s fin structure will have a significant effect on the HVAC / R asset ’ s performance . When HVAC / R coils are coated , they become more resistant to corrosion and other forms of damage , which can help to improve their overall energy efficiency . This is because
Figure 2 : Optical evaluation of filiform corrosion using the Corrosion Inspector – a high resolution line scan camera . Courtesy of Schäfter + Kirchhoff GmbH ( URL : https :// www . sukhamburg . com )
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