Eco Pickled Surface (EPS) is a process applied to hot rolled sheet steel to remove all surface oxides (mill scale) and clean the steel surface. Steel which has undergone the EPS process acquires a high degree of resistance to subsequent development of surface oxide (rust), so long as it does not come into direct contact with moisture. EPS was developed by The Material Works, Ltd., which has filed several patent applications covering the process. It is primarily intended to be a replacement of the familiar acid pickling process wherein steel strip is immersed in solutions of hydrochloric and sulfuric acids to chemically remove oxides.
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The EPS process (see Figure 2) begins with hot rolled strip steel in coil form. This steel pays off of an uncoiler, then passes through a machine which serves dual purpose as a "scale breaker" and "leveler". This machine (see Figure 2) works the material between sets of hardened rollers. This has the effect of removing the curvature of the strip ("coil set") and breaking loose the outer layers of mill scale which encase the steel strip.
After passing through the "scale breaker/leveler" machine, the steel strip enters the first of at least two (and optionally more) "Slurry Blasting Cells". Slurry blasting is a wet abrasive blasting process that combines a fine-particle metallic abrasive with a “carrier liquid” (the most common one being water). This abrasive + water slurry mixture is fed into a rotating impeller which propels it at high velocity across the object to be cleaned (see Figure 3). Slurry Blasting is a method for removing rust/scale, for blast cleaning and shot peening. Cleaning agents can be introduced into the carrier liquid to reduce smut and aid in rust prevention.
An EPS Slurry Blasting Cell is composed of eight of the Slurry Discharge Heads shown in Figure 3 – four for the top surface and four for the bottom surface of the strip. Inside the Slurry Blasting Cells jets of water cleanse the steel strip of both the abrasive particles and the dislodged mill scale. The strip emerges the final Blasting Cell and is dried using high-velocity air blowers. At this point the strip passes between real-time oxide detector modules (one on top, one on bottom), which provide feedback to the line control so full oxide removal is assured. The strip then passes through an in-line, real-time profilometer module which measures surface roughness (Ra) and feeds a signal back to process control to vary impeller rotation speed to produce the desired range of roughness.
To conclude the process, the strip may, optionally, have an oil film or lubricant applied, then it is recoiled. Of note is that tension created by the force of the recoiler pulling the strip through the scale breaker/leveler serves to flatten the strip, removing bow, edge wave and minor coil breaks. Also, not shown in Figure 2 is the slurry delivery/recirculation/filtering system. This closed-loop system collects the carrier liquid, abrasive and removed scale, filters out the removed scale, other contaminants and "undersized" abrasive particles, and returns a cleansed slurry mixture back to the Blasting Cells.
Steel which utilizes the EPS process to remove surface scale shows few differences from steel which utilizes acid pickling to remove surface scale. "Downstream" industrial processes such as galvanizing, cold reducing and painting of EPS-processed steel strip show it to be interchangeable with acid-pickled steel strip. This also holds true for common sheet metal fabrication processes, such as laser cutting, plasma cutting, stamping, welding, bending, and roll forming - no meaningful difference between steel strip utilizing the EPS process and steel strip utilizing acid pickling.
An area where the difference between EPS-processed steel strip and acid-pickled steel strip is apparent is visual appearance. Steel which has undergone EPS processing exhibits a more uniform, lustrous appearance, as shown in Figure 4. In the EPS process, the impact of the abrasive particles on the steel surface serves to "smooth out" minor surface imperfections such as scratches, pits, roll marks and silicone streaks.
Another area of difference between EPS-treated steel strip and acid-pickled steel strip is rust resistance. Conventional acid-pickled steel strip is frequently coated with a thin film of oil to serve as a barrier to contact with oxygen so as to prevent rusting. EPS-processed steel is inherently rust-inhibitive and, therefore, needs no oil or other coating to prevent rusting. Many "downstream" processes and steel fabrication processes must have the steel's oil coating (or other surface contaminants) removed as a precursor step of the process. Use of EPS-treated steel in these processes precludes the need for any such "oil-stripping" precursor step, thereby simplifying the process.
Metallurgical investigations of EPS-processed steel were conducted to determine the mechanism by which the surface is rendered rust-inhibitive. The most conclusive of these investigations was conducted in a reputable metallurgical research laboratory, using highly specialized equipment able to characterize the surface constituents on an atomic level. The conclusion was that EPS-processed steel achieves rust-resistance in a manner that is analogous to stainless steel, although less "robust" than stainless steel.
Stainless steel does not rust because of the interaction between its alloying elements and the environment. Stainless steel contains iron, chromium, manganese, silicon, carbon and, in many cases, significant amounts of nickel and molybdenum. These elements react with oxygen from water and air to form a very thin (roughly 200 Angstrom) stable film that consists of such corrosion products as metal oxides and hydroxides. Chromium plays a dominant role in reacting with oxygen to form this thin film. (In fact, all stainless steels by definition contain at least 10 percent chromium). The presence of the stable film prevents additional corrosion by acting as a barrier that limits oxygen and water access to the underlying metal surface. Because the film forms so readily and tightly, even only a few atomic layers reduce the rate of corrosion to very low levels. The fact that the film is much thinner than the wavelength of light makes it difficult to observe without the aid of the very specialized instrument that was utilized in the investigation of EPS-processed strip.
In the case of EPS-processed steel, trace amounts of silicon, chromium, manganese and aluminum – all constituents of carbon steel at some level – were observed to remain intact on the EPS surface. These trace elements react with oxygen to form a very thin protective layer – a stable "film" – that inhibits further corrosion. The chemical reactions involved in acid pickling remove much of these trace elements from the surface of steel strip, thus a comparable protective film cannot develop.
There is another mechanism at work with acid pickling that tends to invite oxidation. The primary pickling agent is hydrochloric acid (HCl). Although the steel strip is thoroughly rinsed with clean water after immersion in the HCl bath, some residual amount of chlorine (Cl) remains on the surface of the strip. Chlorine reacts very readily with oxygen to form chlorides, so the free Cl acts as something of a "magnet" for oxygen. This mechanism makes acid-pickled steel more prone to picking up oxygen, whereas there is no comparable mechanism at work with EPS mechanical pickling.
Finally, an additive is used in the EPS Slurry Blast carrier liquid to reduce the “smut” that would otherwise remain on the surface and dull the appearance of EPS-processed strip. This additive contains a rust inhibitor, a residual amount of which remains on the surface even after rinsing. It is believed that the presence of the rust inhibitor adds to the overall EPS-processed strip’s ability to resist rusting. The additive has been demonstrated to have no impact on paint performance.
The EPS process produces scale-free steel strip which is interchageable with acid-pickled steel strip, yet the EPS process entails lower capital and operating (variable) cost than an acid-pickling line of equivalent output. For this reason the EPS process is considered to be a direct replacement of acid pickling.
In addition, the EPS process is considered less damaging to the environment than acid pickling for these reasons: