Welcome to the nasty world of intergranular corrosion and stress corrosion cracking. The longer I am in the business, the more I learn, and the more nervous I become.
As most of you know, the original Cobra used some pretty lousy materials in some areas. They were trying to keep costs down. That is common knowledge.
But most surprisingly, and certainly NOT common knowledge, is the fact they used EXTREMELY high quality steel for their spindles and rear hubs. It is a highly nickel modified 4340. In England they call it EN30B. I just call it expensive. It is about the highest strength steel you can get. It is incredibly tough, it is incredibly durable. It is an incredibly lousy choice for long term corrosion protection and who wants their Chariot of Fire to RUST
Now, all steel rusts. Everyone knows that. Just look under your daily driver--be it a Ford GT, a Ferrari Superamerica, or a Pinto. The normal steels used in automotive production rust.
On our earlier cars, we used to make our hubs and spindles out of 4340. Just go to any racing site and they will proudly tell you they make their parts (crankshafts, connecting rods, hubs, spindles, axles, etc.) out of 4340. It is a great material for unbreakable (or almost unbreakable) parts that you are going to use today. If you plan on using them 20-30-40 years from now...maybe ought to try something else. If you live in Florida...you definitely should use something else--unless you like rust.
We have been in business now for 13 years and we are now starting to see some of our cars that have been out to battle for some time coming back for repairs, upgrades, modifications, etc. And we are starting to see rust...I hate rust.
You see, very highly alloyed steels (like EN30B and 4340) will rust if you look at them. (Worse, they even rust when you aren't looking at them...rust never sleeps.) If you take a freshly machined piece of 4340 and put it under hot water, you will literally be able to watch it rust in seconds.
The problem with corrosion is the higher strength the steel, the faster it rusts! WORSE, it rusts even faster under stress! AND WHERE DO HIGH STRENGTH PARTS RESIDE??? IN HIGH STRESS AREAS, OF COURSE!
There just ain't no free lunch (unless you vote for Hillary). So, if you bend the part (like a SPINDLE or a HUB) then the part will corrode FASTER! Starting to get heartburn??? Pass the Tums. It isn't going to get any better any time soon. It is actually going to get worse...
Intergranular corrosion. The most insidious monster of all. Just like the name says, corrosion follows the alloying agents right down the grain boundaries into the part! Oh man. Did you read what I just wrote? Corrosion follows the alloying agents...like chrome, molybdenum, nickel...you get the point yet? I know I got the point--right in my rear end!
If you were to take the above pictured spindle and cut through one of the rust spots you would need to bust out the Depends. The rust spot will actually be BIGGER UNDER THE SURFACE than it is on the surface. (Remember, we have 40 years of corrosion going on here.) I actually did that on an original spindle. I cut it and saw the rust spot get BIGGER--but I have misplaced the spindle or I would post a picture. It is enough to make you weak in the knees.
So,
There must be a better way.
There is...17-4 PH stainless. There must be some disadvantage you say...there is; it is only 4 TIMES MORE EXPENSIVE THAN 4340!!!
BUT
It is extremely corrosion resistant. The surface will rust under really nasty conditions, but it is about as impervious to rust as you can get in these strength levels. We "heat treat" or more accurately "age" our 17-4 parts to H900 (900 degrees) for maximum strength. In the H900 condition they have an ultimate of 190,000 psi and end up in the 40's Rockwell C. I got this tip from a customer who makes parts for the space shuttle. He told me "Half of the parts on the space shuttle are 17-4. I know, because I made them!" Now, I don't know how much of that was hyperbole, but you get the point. The stuff is as bulletproof as you can get...at least in reasonable price ranges. (If you want to talk about unreasonable price ranges, we can start talking about Maraging 250, 300, and 350. But only George Bush can afford that stuff--$30-$60/POUND! Maraging 350 has an ultimate tensile strength of 350,000 psi!!! It is used in missile cases where it can be run extremely thin because it is so strong. Thinner means lighter and lighter means more weight capacity for BOOM to reach out and touch someone.
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