Was there more rust in the early ’70s?

Question:

An article in the March Auto Restorer said something interesting about the range of model years (1971-74) into which the subject Plymouth car fell. The article reads, “Road Runners of 1971 through 1974 rarely did (completely escape rust problems).”

My own casual observation of cars I’ve owned of model years from 1951 through 2008 is that American cars in the precise range 1971 through 1974 had rust problems notably in excess of those even in earlier years, where you would assume rust problems would have been at least as bad, and certainly not better.

It makes me wonder what happened in 1971 (and unhappened in 1974) to make just those years especially rust prone. In any case, I’m interested in the knowledge/experience of any informed expert, such as you.

Answer:

I could not find any data to support or refute the assertion that cars made from 1971-74 were particularly prone to rust.

Having said that, however, it does seem as though cars from the early to mid-1970s did suffer from noticeably worse body rot than both earlier and later vehicles. If this is, in fact, true one possible explanation is the increased use of unibody or partial unibody type construction.

Unibody construction normally creates a lot of enclosed spaces within the body structure and these tend to hold water, dirt, road salt, and other things that promote corrosion. Also, it is quite a bit more difficult to get primer, paint and other anti-corrosive coatings into these enclosed spaces.

A primary advantage of unibody construction over body-on-frame construction is that its increased strength is derived from design rather than an increased quantity of material. That enables the manufacturers to utilize thinner sheet metal for the outer body panels. This has the tremendous benefits of reducing both weight and materials cost, but at the same time it obviously makes the body panels more susceptible to rust-through.

As time marched forward, the automakers and their metal and coatings suppliers continuously improved corrosion protection but maybe the early to mid-1970s represent a time window when unibody construction encouraged corrosion and the technology to counter it hadn’t yet advanced much.