C orrosion: aviations C-wordhas been a problem that A&Pshave battled ever sincedesigners switched from wood tometal. But the concept of avionics cor-rosion is something thats relativelynew because the avionics were work-ing on are getting relatively old. Itcan be bad, especially on airplanesthat are older than 10 years or air-planes that have spent a lot of timenear the coasts, explained JimDurbin, senior engineering managerfor Rockwell Collins. Corrosion ishappening from the time the airplaneleaves the factory. But it may takeyears (to) where it will affect systems(and be) recognized as a problem.

And unlike airframe or engine cor-rosions, which give technicians plentyof visual clues to its location andseverity, corrosion on avionics andelectrical components is extremelydifficult to spot. Plugs, pins, connec-tors, antennal leads and seals are usu-ally located in areas that dont encour-age routine conditional inspections.

Its just a good idea for any avion-ics tech working on an airplane, whenyoure in there getting ready toremove a box, to do a good visualinspection to see if you have any signsof corrosion on, or around, the con-nectors or boxes, added BarryLindsten, director of technical servicesfor Honeywell Aerospace. If you spot

any, thats the time to take appropriateaction to control the problem.

Know thy enemy.But before you can control it you

have to know what it is. Corrosion isthe destruction of metal by an electro-chemical reaction with its environ-ment. If we could view a corrosivecell on a microscopic level, wedsee an effective battery. You have ananode and a cathode, a connection andan electrolyte, such as water, to com-plete the circuitand that circuit iswhat creates the electrochemical situa-tion that causes the surrounding metalto corrode. (If you want a more in-depth look at corrosion and what caus-

56 AVIONICS NEWS FEBRUARY 2003

AS AVIONICS SYSTEMS GET OLDER, TECHNICIANS ARE FINDING THAT THEY HAVE TO ADD YET ANOTHER THINGTO THEIR DIAGNOSTICS AND MAINTENANCE LISTS: CORROSION INSPECTION AND PREVENTION.

B Y D A L E S M I T H

Routinely inspect all plugs, pins and connectors for signs ofcorrosion and contaminant build ups.

Use an approved cleaner and follow manufacturersinstructions for application and removal of materials.

Always have clean plastic bagsand/or plastic connector capsready when removing avionicsfrom an airplane and NEVERremove avionics on the ramp inthe rain.

When storing avionics prior toreinstallation, always keep themcovered and store them in ahumidity controlled room. If theboxes will be in the avionicsshop or on the bench for anextended period of time, makesure to cover them nightly withplastic to keep dust out.

Carefully inspect all rubber sealson avionics access doors thatopen to the exterior of the air-craft for cracks and splits thatcan allow water penetration.This is especially important fornose and radome mounted sys-tems.

T E C H N I C A L

TO O L B OX T I P S F O R AV I O N I C S CO R RO S I O N CO N T RO L

es it, visit the Corrosion Doctors atwww.corrosion-doctors.org, or reviewa copy of FAA Advisory Circular AC43-4A, Corrosion Control for Aircraft.)

The best way to stop moisture andmetal from meeting is to coat the struc-ture with a barrier. Gold and otherhighly conductive and non-corrosivemetals have been used to plate avionicsconnectors for decades. This solutionworks very welluntil the connectionsare subject to the vibration and move-ment of everyday use, which causesthe metals to rub together and wearaway those protective coatings. Thiscreates an ideal setting for the forma-tion of fretting corrosion. And oncefretting starts all bets are off.

The majority of avionics corrosionoccurs at the pin connector at the rearof the box, said Jason Smith, manag-er, Research & Development Lab forZip-Chem Products. Usually it willtake the form of a change in the colorof the pin or connector. It may appeartarnished or in some extreme cases youmay even see rust at the base of theconnector. This means the plating has

degraded to a point where iron oxideappears.

But most times its not that easy tosee, he continued. Even though itsnot heavily spotted, the smallestamounts of tarnish can significantlyincrease the resistivity of the part andt h a t s where you see your failuresbeginespecially intermittent ones.And sometimes visual inspectionswont detect this tiny level of corro-sion.

Making corrosion detection evenharder are those extreme cases wherethe problem is inside the box. Youmay have no visible signs of corrosionoutside the box itself, Lindsten said.But the box could have been exposedto a variety of corrosive elements overtime and they could have migratedthrough to the internal circuitry,between the layers of the CCAs oractually gotten underneath the layers

AVIONICS NEWS FEBRUARY 2003 57

When dealing with avionics thatuse forced-air cooling, inspectthe vents and inside of the boxesfor dust and contaminant buildup. Follow the manufacturerspractices to clean any materialfrom the boards.

Frequently inspect all antennamounts and connectors at theairframe. These areas are highlyprone to corrosion and requireextra attention.

If you decide to apply an appro-priate corrosion prevention com-pound (CPC) follow the manu-facturersinstructions. A littlegoes a long way and over appli-cation will only lead to addition-al clean up.

Make proactive corrosioninspection a part of your compa-nys avionics service proce-dures.

If you ever have a questionabout how to handle any unusualcorrosion situations contact theparts OEM immediately.

A Honeywell weatherradar unit testedunder shower heads.

Continued on following page

of the boards themselves. That can bevery difficult to diagnose and oftenleads to no-fault-foundsituations.

Keeping metal and moisturefrom meeting.

The only way to effectively defeatcorrosion is to keep moistureanykind of moisturefrom meetingexposed metal. Unfortunately, the veryenvironment most avionics live in is avery humid one.

Airplane designers used to putavionics in all kinds of left overspaces, added Mark Pearson, generalmanager of Lear Chemical. Panelmount avionics and remote boxeslocated in the nose of an aircraft areespecially prone to problems. Theseareas tend to create their own humidenvironment. The boxes get hot andthen hold that heat for a long time.Then they have rather long cool-downrates and these conditions promotecondensation to form on and aroundthe boxes.

Moisture intrusion into connectorplugs is a big problem especially whenyou talk about avionics mounted in thenose and radome areas, Lindstenadded. You have a double-whammybecause you not only have water fly-ing up in the area, in winter you alsohave runways treated with salts andthose get up in there too. That stuffreally likes to wick up wires and getinto the connectors which is alwaysbad.

Another environment that can beparticularly unfriendly to avionics isright inside your hangar. Dirt, dust andmoisture are all around and ready toinvade unprotected connectors andplugs. So before you remove a boxfrom an airframe have a clean plasticbag or plastic plug covers handy.

But luckily, plastic bags and plugcovers arent the only weapons in yourcorrosion control arsenal. There are

two steps you can take that are botheasy and inexpensive. The first iscleaning and the second is the applica-tion of an appropriate corrosion pre-vention compound.

Clean parts are corrosion-freeparts.

The proper cleaning of plugs, pinsand connectors can go a long waytowards minimizing the occurrence ofcorrosion problems. Through ourquality practices we train all our tech-nicians to routinely inspect for dirt andcorrosion, Lindsten added. We lookat all contact areas. We also make sureto clean all connectors so that if thereis any trace contaminant on the piece,we remove as much of it as possiblebefore putting it back in service.

Another situation that will frequent-ly create corrosion related problemsoccurs inside the box with avionicssystems that rely on forced-air cool-ing. The problem with forced-air is,when you go into a humid environ-ment, the system will force the mois-ture through the product, heexplained. That moisture will com-bine with any solid contaminantsdirt, dust, human skin particles, youname itand actually create an elec-tronic bridge between the circuitrywhich will start to conduct and lead tofailures on the airplane.

It can also lead to a situation thatwhen the box comes into the avionicsshop, it tests just fine, Lindsten con-tinued, because its not in a humidenvironment. The catalyst that causedthe fault isnt there any more. Thatswhy our technicians routinely cleanthe dust and other stuff out of theboxes whenever they come in. Its asimple step that can greatly reduce theinstances of no-fault-found b o x e sreturning to service.

Actually, cleaning the various com-ponent pins, plugs and connectors is assimple as a quick squirt from anapproved avionics cleaner. Yo u

probably have a few cans in the shopalready. The Mil Spec that weve pat-terned our cleaners after is MIL TRF-29608, Smith added. It wont harmany plastics and is very effective incleaning dirt and contaminants awayand its very fast drying so you wonthave to swab up any residue. But youreally just want to use enough toremove the dirt not flood the area.

One note of caution here is, as of thefirst of January, aerosol cleaners canno longer contain HCFC-141bbecause of its Ozone depleting poten-tial. Because of this some less-expen-sive alternatives can possibly containflammable compounds. So check withyour supplier and read all informationthoroughly before using any newaerosol cleaners on electrical connec-tions.

Better living through chemistry.A&Ps have been using any of a

number of corrosion prevention com-pounds (CPCs) to protect airframes foryears. And its proven to be prettye ffective at keeping moisture andmetal from forming a destructive rela-tionship.

OK, so now weve arrived at possi-bly the most important and certainlythe most debatable question: Shouldyou or should you not use CPCs onavionics? Well clearly the answerdepends squarely on whom you ask.Because it is still very early in thegame, there are strong opinions inboth directions.

In fact, the only extensive opera-tional analysis that Avionics News hasbeen able to find is a report that sum-marizes a study conducted by the U.S.Air Force titled: LubricantE ffectiveness Study for CorrosionProtection and Improved Reliability ofAvionics. The study, performed on150 F-16 aircraft over two yearsfound, Ground based exposures onlubricant test connectors exposeddirectly in all base environments con-

58 AVIONICS NEWS FEBRUARY 2003

AVIONICS CORROSION 101Continued from page 57

firmed two conclusions from earlierwork. One was that gold-plated con-nectors without any protection can cor-rode rapidly. The second was that thebest lubricants can totally inhibit cor-rosion.

One thing I have to point outthough, is that the Air Forces reportonly dealt with the connectors onselected LRUs on the aircraft. The testdid not cover use of CPCs inside of theboxes themselves.

But thats not to say the leadingCPC manufacturers havent beendoing their homework. We have donetesting on all types of avionics sys-tems, Lear Chemicals Pearsonexplained. For example, we did Ohmstesting for Learjet to determine theaffect our ACF-50 would have on theelectrical bond in the aircraft and in theavionics stacks. And what was deter-mined was that a dryconnector and aconnector treated with our product hadexactly the same Ohm resistance.Thats an important finding becauseyou do not want to increase the resist-ance. That would defeat the purpose ofusing the product in the first place.

But it also points to the fact that atechnician has to be careful selectingwhich CPCs to use, he added. Noteverything is appropriate for this typeof application. Currently CPC manu-facturers offer two types of fluid thinfilm products for avionics protection:the oily coatings and the waxycoatingseither of which, accordingto the Air Forces report, are suitablefor use on avionics connectors.

The goal of either type of material isto form a protective barrier betweenthe metal it surrounds and any mois-ture that may be present. Many of thefluid thin film products actually act asmoisture displacers, effectively forc-ing water and corrosives out and awayfrom the metals. Another benefit is thatthe CPCs can act as a lubricant thatkeeps metals from rubbing togetherand promoting fretting corrosion.

So where do you go to find whichcompounds are right for which appli-cation? Again, its back to the MilSpec. The first and best way to makesure a product is OK to use in anavionics application is to go online onthe DODs site (www. d o d s s p . d a p s .mil/assist.htm) and look up whichproducts are on the Qualified ProductsList (QPL), Smith explained. Iftheyre there, these products have beencompletely scrutinized to ensure thatthey meet all performance require-ments. In case you want to do theresearch, the oily products are MilSpec-C-87177 and the waxy CPCsare Mil Spec-C-81309.

Basically the makers of these prod-ucts have had to supply material to anindependent testing facility to confirmthat it meets the militarys guidelines,he added. Its a situation where youdont have to rely on the claims of themanufacturer.

Mil Spec or not, not every avionicsmanufacturer recommends the use ofany CPCs at all. In particular, Lindstensaid that Honeywell does not recom-mend the use of any such compound.It has been our experience that longterm reliability of electrical circuitsand connectors is not improvedthrough the use of such lubricants, andcan in fact, lead to increased incidenceof intermittent failure due to trappedcontamination and wicking into theequipment circuitry, he said.

While the CPC manufacturers cer-tainly cant fault the OEMs, its nosurprise that they dont agree withthem. Weve been on the market forover 17 years and weve been testedon all applications quite extensivelyby avionics technicians around theworld, Lear Chemicals Pearson said.We have had no reports of any nega-tive effects. That combined with ourtesting leaves us totally confident thatACF-50 is non-conductive and cansafely be used on any avionics con-nector.

AVIONICS NEWS FEBRUARY 2003 59

What do you do now?If youve gotten this far and are

feeling rather confused, youre notalone. Like everything in aviationmaintenance you have to approacheach situation as a totally individualand isolated case. If you suspect corro-sion is a cause of a problem, and thebox is under warranty, then by allmeans contact the OEM and followtheir instructions to the letter.

If, on the other hand, the box haslong past its coveragewhich will bethe case in the vast majority of situa-tions, then its up to you to use yourexperience and skill to determine theseverity of the problem and create aneffective solution.

No matter whether you decide touse a CPC or not, no one can argue thebenefits of a careful inspection of allpins plugs and connectors followed byan appropriate cleaning. And dontforget about those antennas. If youare replacing Comms or Navs,remove, check and rebond as part ofthe installation, explained Durbin,Its always bad when a customer haspaid good money for a new radio andit doesnt appear to work any betterthan the old one because the antennahas gotten corroded.

For more information:Honeywell AvionicsCustomer Response CenterTechnical Assistance GroupPh: (602) 436-2005www.honeywellaerospace.com

Rockwell CollinsCustomer Response CenterPh: (888) 265-5467www.rockwellcollins.com

Lear Chemical Corp.Ph: (800) 256-2548 or (905) 564-0018www.learchem.com

Zip-Chem ProductsPh: (800) 648-2661 or (408) 729-0291www.zipchem.com