24ST3 Aluminum VS. 2024T3

[wingnut]

I am not a member yet, and do not own a 170. I own an aircraft repair facility for GA aircraft, 22 years as A&P mechanic and 16 as IA. I should know the answer to the question below, but don't, and would like to draw some knowledge from the older, err more experience gentlemen. Here goes.

What, if any, is the difference between the aluminum, 24ST3 used in older (pre 1960) and modern 2024T3?

It appears that the changeover occurred in the mid and late 50's with most manufactures, I guess until the surplus was exhausted.

My reason for asking is this: My shop has rebuilt/restored four 170's and two 195's in the past 12 months; a noteable increase of aircraft owners spending big money restoring older aircraft. I know this trend started 20 years ago, but I've noticed a considerable accelleration in the past few years. I also notice that the degree of corrosion is much worse on the aircraft using the older designation 24ST, even in the same vintage aircraft. I would like to note however, that the corrosion on the 24ST3, in most cases only effect the surface and rarely do I see exfoliation, intergranular or other types of corrosion. This is in contrast to 2024T3 which seems to resist surface attack more than 24ST3, but I see more corrosion under metal-to-metal joints and exfoliation type corrosion with 2024T3.

These observations have very littile to do with the age of the aluminum, nor the location of the aircraft; 3 of the aircraft had documented repairs when they were young (1, 3, and 11 years old) were I could observe the new skin (2024T3) along side the old skin (24ST3). In all cases the 24ST3 had moderate to severe surface corrosion with very little between metal-to-metal joints, and the 2024T3 had very little surface corrosion and more metal-to-metal.

All examples were unprimered internal surfaces.

[Tom Downey]

The old designation is 24ST3 the new is 2024T-3 but the manufacturing process changed. It is the same material, but how it is made is different.

24= 2400 aluminum alloy
T = treated
-3 = what state the treatment is.

[GAHorn]

T means it is Heat treated (for hardness.) This is usually done in solution to prevent oxidation, and then the metal is drawn to final hardness. (Other designations are "O" for annealed and "F" for as fabricated.) The "S" indicated a wrought alloy. Wrought alloys are less likely to suffer from exfoliation, as you've noted. Modern production methods are designed for speed of production.

Aircraft Spruce has a simple explanation of designations. (I'm not speaking to either wingnut or Tom because they both certainly are very knowlegeable guys already, but anybody who doesn't have an Aircraft Spruce catalog should get one.... and just spend some time reading thru it. It's full of good information about such things for the average guy. It's informative about a lot more things than just the products they sell.)

"Although the old system of aluminum identification has been obsolete for many years, stock with the old markings is still occasionally found. The following comparison is presented as an aid in identifying such materials in terms of the modern system.

In the old system, alloy composition was indicated by a one- or two-digit number followed by the letter “S” to indicate that it was a wrought alloy, i.e., an alloy that could be shaped by rolling, drawing or forging. Any variation in the basic composition was indicated by a letter preceding the numerical alloy designation. For example, A17S was a modification of the basic alloy 17S. In modern terminology these two alloys are designated 2117S and 2017S, respectively. Temper was designated by a second letter: “O” for soft (annealed), “H”for strain hardness of non heat-treatable alloys, and “T”for hardness of heat-treatable alloys. Degree of hardness of non heat-treatable alloys was indicated by a fraction preceding the letter “H”. For example, 3S1/4H would be quarter-hard 3S alloy."

Examples of the old and modern designations of some common aluminum alloys.


Modern System
1100
3003
3003-0
2014
2017
2117
2018
2218
2024T
5052
7075T6

Old System
2S
3S
3SO
14S
17S
A17S
18S
B18S
24ST
52S
75ST6

[N2865C]

George, how in the world do you keep all of that sh*& in your head????

[GAHorn]

George, how in the world do you keep all of that sh*& in your head????

That's why this job pays so well!

[wingnut]

Thanks for the replies George, Tom. I guess I'm really looking for a metalurgist to explain why the noticeable difference in severity of certain types of corrosion with the old vs. new designation. I'm familiar with what the numbers mean now, and that the materials should be the same. It's a curiosity thing for me. Is it the manufacturing process? The clad composition?