Rustproofing and Rust

[emgeetf]

I have a 1963 Healey 3000 BJ7 that was originally rustproofed when new. This rustproofing has held up well for the past 60 years, and still shows that 95% of the underbody has the material that was applied to the frame and the underside of the body when it was new. The original factory green paint is showing in a few spots where the rustproofing is now missing.

Someone told me that despite the rustproofing and the factory paint on the frame, that the frame could still have corrosion and may need repair after 60 years. The car originally came from Kentucky and has been in storage for almost 10 years now. Can anyone comment on this, especially if you had a car that was originally rustproofed and was used in a warmer climate like the south?

The car was driven a few summers after I got it and never drove in the snow, but not sure if it was driven in the wintertime in Kentucky, but I am told that they did not use road salt in the warmer states like Kentucky during the 70s, the time when the car was in Kentucky prior to my purchase.

 

[Bob_Spidell]

There are several types of 'rustproofing,' from chemicals like zinc chromate--green stuff often seen on aircraft structures--to the notorious, tar-like 'undercoating' that is known to trap water and conceal rust as much as prevent it. If you scrape the rustproofing off and see shiny steel the rustproofing did its job, but any penetrating scratch on the tar-like undercoating stuff will trap water, salty or not, which can eventually cause at least surface rusting. Surface rust can be treated with a rust converter and painted over; I've had success with this technique. Suspect areas can be checked with the 'pick' type body hammer, if the pick penetrates, significant repair is required.

Regardless of how the chassis looks, it's a hollow 'box' type and will almost certainly be rusted on the inside. Our BN2 was an otherwise solid car for its age, but when we rotated it on the rotisserie you could hear the rust flakes and powder rolling over inside. There are several holes that should be sealed with a rubber plug, but these are usually perished after a few years and there are plenty of other places water can get inside anyway. Injecting a rust preventative--Waxoyl, Boeshield, LPS3, linseed oil, etc.--into the chassis would have limited internal rusting, but you would have had to do it right off the lot and every year or two afterward. Obviously, rusting compromises the strength of the chassis, but I haven't heard of any breaking in half.

The most likely spot for rust is at the bottom of the 'dogleg' in the front of the rear wheel wells. It's a pocket that traps debris and water and though it should have a drain hole, it's often plugged.

 

[WHT]

Bob has a good discussion regarding the condition of most original Austin-Healey frames after 56 plus years of use, and recommendations for additional corrosion prevention..

The main problem is caused by the thickness of the steel used in the Healey frame, its yield strength and the welded "C-section" frame tube construction. The frame undergoes considerable flexing when driving because of the thin tube walls and somewhat weak steel; and the flat, welded seams of the welded "C-section" frame rails eventually open and allow moisture to enter.

Below is an analysis of the original frames from a metallurgical engineer (David Seib):


"The original Austin-Healey frame was fabricated by welding two 0.072-inch thick “C” channel stampings together to form a box shaped tube. The welding of these stampings was performed using longitudinal edge welds. These edge welds were most likely selected for their significant fabrication advantages, which include: lower tendency to warp, ease of inspection and ability to hide misalignment. The fabricated box shape tubes (i.e. rails) had two flat parallel surfaces on the top surface, with ¾ inch height transition at about the rear shock towers. The bottom surfaces of these main frame rails display constant bow. Maximum tube height, 4 inches, occurs at about the forward outriggers. Tube height at the forward and aft ends of the car are 3 inches and 2 inches respectively.

Donald Healey recognized torsional stiffness contributed to handling and specified that the frame was to have torsional stiffness equivalent to the Nash-Healey N type. He also specified that the frame was to have a maximum allowable frame bending stress which he provided. Unfortunately, this value is not known.

Independent testing showed that the maximum bending moment on the main frame rails is about 747 feet-pounds, and is located at the junction of the forward outriggers. Incorporating the varying box section height of the frame rails as a function of length, maximum bending stress along the bottom surface of the frame rails at the junction of the forward outriggers is 7,630 PSI (weak).

The steel used has a yield strength of approximately 45,000 PSI and the ultimate tensile strength is approximately 50,000 PSI. These values indicate that the original safety factor to prevent yielding of the frame is about 5.9. A book by Mr. Forbes suggest a static safety factor of 6 was typically used in frame design at that time.

Review of current American Institute of Steel Construction (AISC) and American Welding Society (AWS) codes suggest, however, that the static safety factor may not be this high. For box tubing of the 3 x 4 inch size, the AWS code specifies a minimum wall thickness of 0.109 inches (not 0.072 inches as in the Healey frame). The AISC minimum wall thickness per code is 0.085 inch. The AISC code is probably the more accurate of the two, as it was significantly overhauled in 1961 to better account for buckling of thin walled tubing. Note, this was 10 years after the Big Healey was designed. This means that the 5.9 static safety factor for the original frames may not be achievable due to compressive buckling of the tube at lower stress."