tire pressure loss

I have a 1992 Accord ex that is loosing pressure in one of the tires. I fill the tire to 28psi the next day I'm down to 24psi, next day 22psi this continues until about 18psi and then the pressure loss seems to
stop.The tire has been removed and the mechanic says it's ok. I have noticed there is a lot of corrosion around the rim of the wheel next to the tire. Could this be the cause of the problem? If so, what should I do?
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rfarrar wrote:

BINGO! The corrosion buildup on your rims is likely the cause. I've run into the same problem on several vehicles - all with alloy wheels.
The cure is to have the tire dealer remove the tire, remove the corrosion with a wire brush or power wire cup brush, remount the tire with a goodly amount of bead lube, rebalance and give it a couple years until you have to repeat the cycle.
It's not universal, I'm told but prevalent enough with the alloy wheels that the dealers know exactly what to do.
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I've read in the newpaper's mechanic column that you need to reseal the aluminum with a clear lacquer;that most aluminum castings are porous and will leak air if unsealed. It also prevents the return of the white corrosion.
Using nitrogen for the fill will further slow leakage. I've read where more shops are now offering this.One I saw offers free refills.
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Jim Yanik
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Jim Yanik wrote:

Heard that about the porosity of cast wheels as well.

Any mention of why the nitrogen is used? Does it slow the leakage (hard to figure that) or more that it slows the formation of corrosion?
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http://www.automotivedigest.com/view_art.asp?articlesID 827
Significant Points 1. Proper inflation is key to improving fuel economy up to 3% whatever used to fill 2. Nationwide fewer than 10% of tire dealers offer nitrogen but number growing 3. Finding way into mainstream at growing number of tire dealers including Costco 4. Most dealers charge $2-$5/tire for initial nitrogen fill-up, generally offer free lifetime refills 5. Improved pressure maintenance important for sensitive automatic monitoring systems
Background 1. Nitrogen molecules bigger than oxygen molecules so seeps out more slowly 2. Nitrogen resists heat buildup better than air which contains moisture 3. Nitrogen reduces oxidation that can damage tire from inside out 4. Nitrogen is inert gas so no safety or environmental issues
From http://www.mfgdist.com/aircel-n2cel.htm ; Nitrogen maintains tire pressure better.
Oxygen in air is a tire killer. Compressed air contains 78% nitrogen and 21% oxygen. Oxygen in compressed air is very aggressive and reacts chemically with rubber causing it to lose its elasticity and strength. Oxygen first consumes the tire liner, then ravages the insulating rubber as it permeates through the entire casing. This deterioration is accelerated due to the pressure difference between tire pressure and atmospheric pressure. In testing, tires inflated with nitrogen lost only 2 psi in a six-month period, whereas tires filled with compressed air lost six times as much in the same period. Since nitrogen maintains tire pressure much longer and will not react with rubber as oxygen does, tire failure is cut dramatically. With more consistent tire pressure and improved tire durability, handling for those larger loads is better, too, making your drivers safer drivers.
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Jim Yanik
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rfarrar wrote:

Then the mechanic is a fool. A tire which is loosing that much air will certainly show itself in a bubbling water test if said mechanic knows what they are doing.
Find someone competent.
John
PS Yes, rim corrosion can cause air leaks at the bead and at the valve stem.
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