Variation In Salt-Spray Results
Our chemical supplier is telling us that variations in steel from different mills can produce huge differences in salt-spray results even though it is all cold rolled commercial quality steel. How much truth is there to this, and is there anything specific to look for, or is it a matter of doing extensive testing with materials from various mills?
Q. We recently did some salt-spray testing on various different panels. We tested on Bondrite B1000 panels, ACT cold rolled steel unpolished panels, and cold rolled steel that we use for the manufacture of our customers’ parts purchased from various steel suppliers.
The Bondrite panels, which of course are not run through our pretreatment process, performed very well. So did the ACT panels, which were subjected to our in-house iron phosphate pretreatment.
However, the test panels cut from our in-house cold rolled steel did not perform very well at all. These were run through our pretreatment system along with the ACT panels. The in-house panels were cut on a laser using nitrogen, and the only oil on the steel was that from the steel mill.
Our chemical supplier is telling us that variations in steel from different mills can produce huge differences in salt-spray results even though it is all cold rolled commercial quality steel.
How much truth is there to this, and is there anything specific to look for, or is it a matter of doing extensive testing with materials from various mills? Thanks for your input. B.H.
A. I think there is certainly truth in what your chemical supplier is telling you. There are several variables that I can think of in the steelmaking process that could affect the outcome of your pretrement and paint results.
The first is the steel alloy itself and its heat treatment. Not all cold rolled, mild steels are the same. One of the primary differences could be the carbon content.
Varying carbon content and heat treatment would be two potential differences between the materials. Of the standard panels I have in-house, one lists the steel alloy as 1008, which is a low-carbon (0.10 wt% maximum) mild steel. A higher carbon content, along with varying thermal history (both unintentional from processing and intentional from hardening, annealing, etc.) can cause significant variation.
Additionally, deleterious elements such as sulfur will have a negative impact on corrosion resistance. Different manufacturers with differing mill practices will produce steel with varying levels of some of these elements, leading to more variability in corrosion test results.
Another potentially large source of variability is the type of lubricants and rust preventatives used by the steel manufacturer. The process lubricants will generally be soluble oils that will act as coolant and provide lubricity. Lighter oils may be used in finish rolling.
Finally, a rust preventative would be used to minimize corrosion of the steel in storage. This would likely be one of the more highly variable factors of the incoming steel, since the intent is to get it delivered to you without rust.
However, there are three factors that will go into the choice of rust inhibitor to meet this goal. They are packaging method, transportation and storage time/location.
The more extensive the packaging, the lighter lubricant that could be used, possibly in conjunction with a volatile corrosion inhibitor (VCI) or oil fog that displaces water from the surface of the steel. Less packaging will require more rust preventative.
However, the varying amount of storage time and transportation methods would require significantly different rust inhibitors. If a coil of steel is produced in China and shipped by boat to the U.S., it will also require a much better rust preventative than if it were produced here and shipped directly to you.
Typically, there are several other steps that may be performed by a steel service center, such as rerolling, slitting and possibly further heat treatment, unless you buy such a large quantity that you purchase direct from the mill.
Last would be the storage time at various locations. Again, if you are directly sourced from the mill, this is the shortest route you can take. However, there are inevitable delays that likely occur through processing at a service center and holding the material in inventory on your shop floor.
In the end, it is possible this rust preventative is formulated to provide 9–12 months of indoor corrosion protection. That’s going to require a good cleaning process to remove. Some outdoor steel rust preventatives are even more difficult to remove and take on the consistency of a wax. Generally, the “better” the rust preventative, the longer the storage time afforded and the more difficult it is to remove.
The last important factor is surface finish. Generally, the rougher the surface finish from the mill, the more likely corrosion will be to occur, all other things being equal. This is a function of how the material is ordered, since there are specifications available to deliver the desired finish quality.
Based on all the factors above, it is understandable how much variability you may expect from your incoming cold rolled steel. Your iron phosphate pretreatment system has its own limitations and cannot be expected to level out all that potential incoming variability.
It will be important for you to find a supplier of steel that works well with your pretreatment system, and then work at specifying as much as you can about the process such that you can keep it locked down and providing a consistent quality finish.
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