Have you looked at your car lately? Most likely, other than needing washed and a coat of wax, the body is rust free. Most car owners, including me, take this pretty much for granted today, but just a couple of decades ago, we were always on the lookout for the tell tale signs of blisters in certain areas of the car body where rust was beginning. It's amazing what the auto industry worldwide has done to keep car bodies looking like new-from 6 to 10+ years.
So what, you ask? I make note of this because of seeing an obituary for Nicholas Ross who passed away in January 2004. Nick spent 40 years with then Ajax Magnethermic Corp. (Warren, Ohio) and then started his own consulting company in 1986. Among the many technical achievements of Nick Ross was his invention of the induction galvanneal process patented in 1988, which revolutionized the use of this coating technology to hinder corrosion on auto sheet steel, as well as other applications. Most of the automobiles made in the world today contain a large percentage of galvanneal steel manufactured using the induction galvanneal process.
Coating with zinc (galvanizing) is one of the most widely used and cost-effective means to protect steel against corrosion, and has been used for more than a century. Conventional hot-dip galvanizing is the most commercially and universally accepted method of galvanizing steel. Molten zinc is bonded into the surface of the steel, creating a thicker, ductile layer of "free," or pure, zinc on the surface and a very thin alloy layer of zinc and iron between the free zinc and the steel. Galvanized steel is superior to bare or painted steel. However, a disadvantage is that paint does not adhere very well to the galvanized product (free zinc in particular) unless it is pretreated, which generally is not cost-effective for a majority of applications.
Galvanneal strip is the paintable alternative. Hot dip galvanizing is used to produce galvanneal strip, where the plain zinc coating of the traditional galvanized product is converted to a zinc-iron alloy. Galvanneal strip is produced by reheating the strip after it emerges from the coating bath, wherein the necessary coating composition develops during the thermal cycle. The layer of "free zinc" essentially is converted into an iron-zinc alloy, which extends all the way to the surface of the steel. The result is that the final product has a coating that is an alloy of approximately 90% zinc and 10% iron. The resulting product retains the desired galvanic (electrochemical) protection while gaining other characteristics particularly important to the auto industry including better resistance welding performance than plain zinc coatings and-most notably-excellent paint adhesion without the need for expensive pretreatment (etching, shot-blasting, etc.).
Reheating the strip was carried out in a gas-fired furnace, which had some process problems including the gas jets disturbing the molten zinc and creating localized hot spots. The development of the induction heating process allowed achieving the necessary thermal cycle much faster with a more uniform product.
This increase in uniform product quality led to the significantly longer vehicle life we have today.