After reviewing the results of a battery of paint performance and welding tests conducted on EPS samples, the US automotive OEMs General Motors and Chrysler have approved the use EPS-processed steel as a replacement of acid pickled for end use product application.
|The tests were sponsored by an EPS Producer and long-time supplier of flat rolled steel to automotive OEMs and Tier 1 suppliers. They were conducted over the course of 2013 and the results have recently been shared with TMW. We have incorporated them into our compilation of EPS laboratory and manufacturers studies, EPS END USE AND APPLICATION TEST RESULTS >>.|
All paint pretreatment, appearance and corrosion tests were performed by the accredited testing laboratory ACT of Hillsdale, Michigan. The welding research and testing firm Applied Engineering and Integration, Inc. (AET Integration) performed the resistance spot welding tests and analyses. All tests were performed in accordance with the relevant GM and Chrysler standards. For the paint appearance and corrosion tests, all samples were cleaned, pre-treated and painted in accordance with automotive exposed panel standards.
Specific tests reviewed by General Motors consisted of:
|Paint Appearance and Performance:|
• BYK Wavescan
• Profilometer Surface Roughness
• Stone Impact Resistance
• Corrosion Rating Scale
• Corrosion Scribe Creepback
• Cyclic Corrosion Test
|Spot Resistance Weldability:|
• Weld Lobe Generation
• Electrode Life Test
• Shear Tension Test
• Cross Tension Test
• Metallurgical Examination
• Microhardness Test
• Cap Life Test
To review the GM Test reports >> Click Here <<
The specific tests reviewed by Chrysler consisted of:
• Phosphate Macro-Appearance
• Phosphate Crystal Size
• Phosphate Coating Weight
• Adhesion Testing
• Shear Testing
To review the Chrysler Test reports >> Click Here <<
EPS Difference from HRPO: Phosphate Pretreatment
One of the few of these tests where the EPS samples were tested side-by-side with acid pickled (HRPO) samples was the Chrysler Phosphate/Pretreatment tests. The results of that particular test showed a fairly consistent difference between the EPS samples and the HRPO phosphated samples:
- the phosphate coating weight (mg/ft2) on the EPS samples was lighter than the HRPO;
- the phosphate crystal size for the EPS samples was smaller than for the HRPO samples (see SEM scans below).
At first glance, the fact that less total phosphate coating was deposited on the EPS samples might appear to be a cause for concern. But on closer inspection, the Scanning Electron Microscope (SEM) 1000X magnification scans reveal a smaller crystal size and overall ‘tighter pattern’ of phosphate deposition on the EPS samples. This turns out to be an advantage for EPS: As a general rule, a smaller crystal size / lower weight coating will exhibit better adherence than greater size/weight coatings.
However, a smaller crystal/lighter coating is also thought to offer lower corrosion resistance than larger crystal/heavier weight coatings. Is this a concern for EPS? In the Salt Spray Cyclic Corrosion tests conducted as part of the GM tests, the painted EPS samples achieved a the highest possible rating of “10 – No Visible Corrosion” on the GMW15356 Rating Scale. Also, in Salt Spray Cyclic Corrosion Tests conducted in 2006 according to the same General Motors standards, EPS had achieved that same “10 – No Visible Corrosion” rating as well. (Note that these earlier tests were sponsored by an EPS User and not submitted to GM for review/approval).
The bottom line is that EPS phosphate pretreatment has the potential to offer better paint adhesion than HRPO, with no sacrifice in paint corrosion resistance. Plus, two important automotive OEMs are aware of this and have approved EPS as a replacement for HRPO.