��������

By Cynthia A. Gosselin, Ph.D., The ChemQuest Group

In the simplest terms, sustainability could be defined as less waste and more efficiency. This could mean fewer processing steps, the use of fewer chemicals or other building blocks, or the ability to develop products that last hundreds of years without the need for constant maintenance.

In the coatings (paint) world, sustainability is usually looked at through the lens of fewer chemicals, biomaterial substitutions, shortened or fewer manufacturing steps, or outright chemical bans. Significant efforts are currently underway to reduce petroleum-based materials in the first place along with trying to make the new products last at least as long as their predecessors.

There is another way to look at materials sustainability. What if the pretreatment chemicals and coatings could be eliminated altogether? What if the oxide was also the color coat? What if the longevity of stainless steel could be exploited together with its oxidation potential? What if color could be imparted onto stainless steel strip using only one production operation for both steelmaking and coloring? What if these innovative ideas were actually more cost effective?

First Steps

In 1971, Allegheny Ludlum Steel Corporation took a step in that direction. While certainly not under the guise of sustainability, the company developed a process that produced black stainless steel for use in formed architectural products.¹ In this case, a stainless steel strip was produced by oxidizing the stainless surface to form a porous oxide coating and impregnating it with an alkali metal silicate, and then drying and fusing the silicate by heating between 1400 and 1600 °F. Temper rolling the finished product—a typical steel finishing process—allowed for flattening the strip to facilitate fabrication.

This process was an efficiency improvement over the original pre-1968 process where many more production steps were used. First, the strip was treated to obtain a black oxide and then coated with a water-soluble alkali metal silicate or a water-based thickened or gelled solution of sodium or potassium dichromate. The next step involved removing the water and subsequently heating the coated surfaces to a temperature high enough to form a uniform, black, porous oxide 1,000–500,000 Å thick. Excess coating and loose oxide were then scrubbed off with water.

In addition to reducing the number of processing steps, the 1971 process eliminated many quality problems. There was better thickness uniformity, less visibility of any spray or rolling pattern, good aesthetics, and better condensing humidity resistance, as depicted by the absence of white residue all over the strip. Steam testing, abrasion resistance, chemical resistance, mandrel bend, and impact testing were also improved to the point that this black stainless could be formed into tubes or building panels with decent aesthetics. With better oxide thickness control, color was also more uniform.

Fast Forward to 2013

Over the 40 years since the first steps in manufacturing blackened stainless, many material inventions and improvements have occurred. Stainless steel itself has become a more uniform product, with improved surface finishes that can now be used in a variety of aesthetic-critical applications. Base metal chemistry modifications and more controlled annealing atmospheres and temperatures have led to increased formability as well.

Alongside steel production maturation, the advent of affordable, usable nanotechnology opened a huge window for surface modification. Nanoparticle surface treatments were developed to reduce damaging oxidation and corrosion of stainless steel and other alloy components in oxidating and corrosive conditions.²

Improving the longevity of stainless steel at elevated temperatures was the genesis of this development. Power generation plants often have problems with damaging oxidation from accelerated high-temperature fire-side corrosion due to molten alkali salts. Other problems include accelerated medium-temperature fire-side corrosion due to low oxygen activity environments and the presence of sulfur and steam side oxidation of tubing, piping, and valves in fossil fuel-fired boilers.

Pressure has been placed on power plants to increase efficiency, meet stringent environmental regulations, and ensure plant reliability, availability, and maintainability—ideally while minimizing costs.³

Some of the nanoparticles used as surface treatments for corrosion and oxidation resistance include aluminum, silicon, scandium, titanium, yttrium, zirconium, niobium, lanthanum, hafnium, tantalum cerium oxide (nanoceria), and thorium. These nanoparticles do not really form coatings but rather are doping agents that become embedded within the oxide structure.

Over time, yttrium has become a preferred nanoparticle for these types of applications. Yttrium, particularly in the form of yttrium oxide nanoparticles (Y2O3 NPs), is considered superior because of its exceptional properties around high thermal stability, strong chemical stability, mechanical strength, and strong corrosion resistance. Perhaps the greatest attribute under the veil of sustainability is that yttrium has low toxicity. Because of its low toxicity, this nanoparticle is often used in biomedical applications.

Minimox® is one of the yttrium-based, self-protective alloy treatments that minimizes oxidation of alloys at elevated temperature. It changes the structure and/or chemistry of the thermal oxide, which becomes denser and more adherent than thermal oxide without Minimox® doping. Flaking, protrusions, and microvoids are eliminated, resulting in a smooth, dense, adherent surface.⁴

Depending upon the time and temperature of the annealing process, thin film oxides can be grown to thicknesses between 500 and 9,000 Å (1 Å = 0.1 nanometer). Oxidized surfaces doped with yttrium are characterized by excellent cyclic and isothermal oxidation, corrosion resistance in severe environments, and high resistance to severe heat and chemical environments. As a bonus, the thickness of the oxide can be controlled within a high-temperature annealing atmosphere to purposefully impart color onto a metal surface.

Modern Colored Steel

The black stainless steel for architectural applications referenced earlier in this article did not result in large volumes of product released into the field. Despite the excellent properties derived from the surface modification, stainless steel was too expensive for run-of-the-mill building applications. However, the idea of a black surface that did not require painting simmered in the background for many years. The thought of being able to colorize a surface using existing in-line annealing practices was a tantalizing thought. Black stainless manufactured using the existing in-line annealing process, without the addition of either paint or doping agents, was successfully accomplished in 2018.⁵ However, stainless remained an extremely expensive substrate for many applications.

It was determined that other substrates that were more in line with cost constraints of industries, such as appliance and automotive, would also benefit from this oxide modification methodology. Electrogalvanized steel (EG) was textured to the required product finish and the zinc coating was subsequently annealed. The resulting black substrate met all the requirements of appliance finish specifications for refrigerator door applications (the most stringent requirements in the appliance world). This “faux stainless” EG product was compared to black polished stainless and the GE Black PVD standard, and it was found to meet all requirements, including color (Figure 1), gloss, stain/aging, anti-fingerprinting, corrosion resistance, and mechanical properties—all at a much lower cost.

FIGURE 1 Black color comparison between standard product, black stainless steel and “faux stainless steel” textured electrogalvanized.

Finally, in 2023, a method of colorizing stainless steel using yttrium doping and annealing was developed. To prove the concept, stainless steel coils were coated with an aqueous yttrium pretreatment solution and batch annealed for several hours to oxidize the surface. Batch annealing is not particularly uniform, and is not the streamlined process of choice, but enough of the coil exhibited the required properties to move forward and optimize the process.

The basic idea was to dope the surface with an aqueous yttrium nano-solution using an in-line roll coater, followed by continuously annealing the strip while varying time and/or temperature in the annealing furnace to grow the oxide and obtain the desired color.⁶ Even keeping the annealing furnace atmosphere constant, varying line speeds (time in anneal) can allow for the efficient production of different colors. (See Table 1.) These colored stainless coils were fabricated and placed on structures in the Nashville, TN, area (Figure 2) and are still in place today.

FIGURE 2 Colored stainless panels on structures in the Nashville, TN, area.

All the aforementioned work has culminated in less waste and more efficiency. Technological improvements in substrate quality, steel processing, and nanotechnology have bolstered the development of a variety of products previously deemed impossible, too expensive, or of insufficient quality for commercial production. By looking at outside-the-box alternatives, it is possible to obtain color by manipulating the oxide and removing the organic coating completely.

References

1. Helgert, Harold. L; et.al. Method of Making Black Stainless Steel Sheet. U.S. Patent 3,556,871, January 19, 1971.
2. Kerber, Susan J. Nanoparticle Surface Treatment. U.S. Patent 8,568,538 B2.
3. Stott, F. H. Influence of Alloy Additions on Oxidation. Materials Science Tech. 1989, 5 (8), 734.
4. Material Interface Inc. Protecting Furnace Components with Minimox® Alloy Treatment. Minimox® Product Data Bulletin. 2016.
5. Myers, F.A. Black Steel. AK Steel Presentation, 2018.
6. Myers, F. A.; Price, L. R. Method of Colorizing Stainless Steel Using Strip Anneal Processing. U.S. Patent 11,584,997 B2, February 21, 2023.

Cynthia A. Gosselin, Ph.D.,��is director at The ChemQuest Group, www.chemquest.com. Email: cgosselin@chemquest.com.