Aluminum Fuel Tank Corrosion

[Agent K]

My team and I have recently completed our initial designs and are fielding initial quantities of our system to theater in the Middle East (Desert Climate). These systems require an auxiliary fuel tank which was designed custom for this application to hold up to 55 gallons of JP-8 fuel. After almost a year deployed in the field, we are beginning to see some serious corrosion issues at the base of the fuel tanks. Some Specs are listed below:

A. 1/2-14 NPTF (STAINLESS) FEMALE 90 DEGREE PICKUP FITTING WITH
ALUMINUM GUY TUBE AROUND NYLON TUBE.
B. 1/2-14 NPTF (STAINLESS) FEMALE 90 DEGREE RETURN FITTING WITH
ALUMINUM GUY TUBE AROUND NYLON TUBE.
C. DOT APPROVED AND BE CONSTRUCTED OF .188 ALUMINUM SHEET.
D. FULLY WELDED SEAMS AND TO BE 100% PRESSURE TESTED.
E. .250 THICK ALUMINUM LIFTING BRACKETS.
F. BUILT-IN BAFFLES TO REDUCE SLOSHING, NOT WELDED TO THE BOTTOM.
G. NON-LOCKING GAS CAP.
H. STAINLESS ROLLOVER VALVE (PRESSURE RELIEF VENT).
I. .375 FTP DRAIN PLUG.
J. MECHANICAL FLOAT FUEL GAUGE.
K. 55 GALLON CAPACITY.
L. TANK CLEAN OUT.
M. STATIC GROUND POINT USING A CLEARANCE HOLE FOR Ø .25 BOLT.
N. BOTTOM PLANE FLATNESS OF ±.06 AFTER PRESSURE TEST.

These tanks are powder coated and have a foam cushion installed at the base for stabilization. I really would like to hear input on likely causes of corrosion this severe as well as possible remedies that have been successfully implemented in extending the life of coated aluminum.

 

[3DDave]

What's the alloy?

 

[IRstuff]

The pattern of corrosion is round, suggesting that there are pinholes in the paint.

Have you opened one up to see inside? Seems to me that would be a first step. The interior might be way worse than what you can see on the outside. Are these units used in a salt environment? High temperature and salt are double bad.

TTFN (ta ta for now)
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[Agent K]

The Alloy is 5052 H32

I have not been able to get one of these corroded tanks shipped back overseas at the moment, but they are positioned on land at a small island (less than 500 sq. mi) in the middle east and surrounded by a salt water gulf. So salt almost certainly will be a factor. I have very little experience with aluminum corrosion prevention and am starting to wonder if having these tanks powder coated served no purpose other than aesthetics at this point. Hopefully I can get one cut open by a deployed service rep overseas in the next few days and provide more information in that regard.

 

[WKTaylor]

Hmmm... Sounds like Diego Garcia...

WARNING.
Desert-seacoast and tropical-oceanic island environments are THE MOST CORROSIVE I have ever encountered! Salty-humid-warm air pervades these environments... and there is often precious little rain or condensing-moisture to rinse off the salty deposits for long stretches of time

All aluminum alloys require an inorganic surface coating, for organic coating adhesion promotion, be applied BEFORE application of a corrosion protective primer and decorative topcoat. This multi-layer protection scheme is vital for aluminum environmental longevity. Alloy 5052-[any condition] is NO exception. IF all You applied was a powder-coating to the bare aluminum, I guarantee You, coating adhesion was destined to be very poor and possibly 'spotty' depending how well the bare surface was cleaned.

NOTE.
After application/cure of the paint system did You accomplish random 'pencil hardness', 'dime-scratch' and 'wet-tape' testing to verify proper paint toughness [cure-state] and adhesion?

Compounding this situation are 'the well-intended padding-strips' around the base. Since the tank sits tightly on these padding-strips it is likely they get saturated with salty moisture [air and precipitation and possibly wash-water] that would permeate into the open space under the tank bottom. Conversely, the [black foam rubber or Felt?] padding strips are arranged [around the bottom of the tank] 'butted end-to-edge'... which appears to form a barrier to 'natural air circulation' that would normally keep the bottom mostly dry... but in this case probably retains the moisture for long/indefinite periods

Some useful references...

https://quicksearch.dla.mil/qsSearch.aspx

MIL-HDBK-509 CLEANING AND TREATMENT OF ALUMINUM PARTS PRIOR TO PAINTING
MIL-STD-193 PAINTING PROCEDURES AND MARKING FOR VEHICLES, CONSTRUCTION EQUIPMENT, MATERIAL HANDLING EQUIPMENT, AND SPARE PARTS
MIL-T-704 TREATMENT AND PAINTING OF MATERIEL

USAF T.O.s

https://www.robins.af.mil/About-Us/Technical-Orders/

1-1-8 APPLICATION AND REMOVAL OF ORGANIC COATINGS, AEROSPACE AND NON-AEROSPACE EQUIPMENT
USAF T.O.
1-1-691 CLEANING AND CORROSION PREVENTION AND CONTROL, AEROSPACE AND NON-AEROSPACE EQUIPMENT
35-1-3 CORROSION PREVENTION AND CONTROL, CLEANING, PAINTING, AND MARKING OF USAF SUPPORT EQUIPMENT (SE)

Regards, Wil Taylor

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[Dhurjati Sen]

Dear Agent K,

The corrosion seems external, the pits have started forming from outside.

It seems to be just a matter of poor painting. Catch the painting contractor and ask him to compensate. If that is not to be, blacklist them for good.

Regards.

DHURJATI SEN
Kolkata, India