A cost-effective metal-cladding process

Dr. Neil Canter, Contributing Editor | TLT Tech Beat December 2011

Corrosion-resistant metal alloy bonds to a second metal substrate that is vulnerable to corrosion. 

KEY CONCEPTS
• In a metal-cladding process, a corrosion-resistant metal alloy coating is bonded to a second metal substrate that is vulnerable to corrosion.
• Current metal cladding techniques such as welding are inefficient and costly.
• A new high-energy-density fusion cladding process has been developed that can be done more efficiently at a lower cost and with a widerange of metal alloys.

Efforts continue to find effective ways to prevent metals from corroding in very aggressive environments. Conditions that metal face include high alkalinity, abrasion and acidity.

Learning more about how corrosion starts is essential to developing more effective measures to stop it. In a previous TLT article, a new instrument was discussed that evaluates corrosion at the nanoscale (1). The instrument is an electrochemical cell that is an accessory for an atomic force microscope (AFM). Electrochemical experiments can be done on a specific surface and images of the changes occurring can be taken at the same time to show the extent of corrosion.

Another approach to minimize the onset of corrosion is cladding. Mario Medanic, general manager, cladding at MesoCoat, Inc., in Euclid, Ohio, says, “Cladding involves the preparation of a thick, metallurgically bonded coating between two different metal alloys. The alloys used, cladding thickness and degree of cladding needed are dependent upon the application.”

In cladding, the bonding takes place between a corrosion-resistant alloy and a vulnerable metal alloy substrate through an electrical or mechanical procedure under high-temperature and high-pressure conditions. For the most part, the substrate is a ferrous alloy that is used in an environment where it will corrode. The corrosion-resistant alloy is typically an aluminum, nickel or stainless steel alloy.

There are a number of techniques used in cladding a metal surface such as explosion bonding, forging, laser welding, MIG welding, TIG welding, PTA welding, friction welding and coaxial extrusion. All of these cladding procedures have several limitations such as slow processing and cost. In the latter case, cladded steel can cost as much as five times more than carbon steel.

This factor has prompted end-users to look at other metal-finishing processes such as fusion bond epoxies, which are not even close in performance to cladding but are readily available and cheaper. There is need for a more efficient cladding process that is also more cost effective. Such a process has now been developed.

HIGH-ENERGY-DENSITY INFRARED ARC LAMP
MesoCoat has developed a high-energy-density fusion cladding process that is more effective in applying metallurgical cladding to a metal surface. The process is known as CermaClad.™ Greg Engleman, chief technical officer for MesoCoat, explains, “We utilize a high-density infrared (HDIR) arc lamp to rapidly increase the temperature to a point just below the melting point of the metal substrate. This enables the metal cladding to be effectively bonded to the substrate.”

In the first step of the process, the metal substrate is prepared by conventional surface-cleaning methods. Several techniques such as a spray gun are then used to deposit the atomized metal alloy slurry on the substrate. This step is followed by the use of the HDIR lamp to bond the metal cladding to the substrate (see Figure 2).


Figure 2. Use of a high-density infrared arc lamp enables cladding to be done more efficiently while retaining the mechanical strength of the metal substrate and increasing the application rate. (Courtesy of MesoCoat, Inc.)

The HDIR lamp melts the alloyed metal slurry almost immediately at temperatures in excess of its melting point. This step is possible because the plasma energy generated can be focused on the surface, enabling the alloyed metal to efficiently fuse to the metal substrate.

A key benefit for this cladding process is that the heating can be controlled so that the mechanical properties of the metal substrate can be controlled. In contrast, other methods such as welding create a large heat affected zone (HAZ) at the substrate metal causing it to lose structural integrity.

With other cladding techniques, the metal substrate will tend to stir up during the process, which necessitates the use of a higher amount of cladding material that reduces the corrosion protection. A longer timeframe is also needed for these other cladding procedures. The use of the HDIR lamp is more efficient in that less stir up is seen with dilutions as low as 1% at 1 mm from the bond line. This leads to the need for only a thin coating of the metal alloy, which minimizes the amount of expensive alloys used and time needed to do the cladding process.

Application rates using the HDIR lamp will increase to hundreds of pounds per hour, which is a factor of two increase over weld overlay and laser cladding. Uniform coatings can be applied over a range of surface areas. In fact, the HDIR lamp enables tracks between 100 and 300 millimeters in width compared to the 15-25 millimeter-wide tracks obtained from weld overlay and laser cladding.

Anupam Ghildyal, senior business associate for MesoCoat, says, “The main applications for cladding are in the oil and gas industries. A key use is to protect steel pipes used in production and transportation of oil and gas in deep-water applications. Other uses involve strengthening infrastructure such as bridges and in ships.”

The HDIR cladding process can use a wide range of metal alloys to bond to substrates, including stainless steel and inconel. Engleman says, “Two alloys that we have specifically targeted for substrates are 625 inconel and 316L stainless steel. Theoretically, there is no limit to the types of alloys that can be used to coat substrates.”

Currently, the HDIR process is being scaled-up. Ghildyal says, “We anticipate commercializing this process during the second quarter of 2012.” R&D Magazine has recognized the HDIR Technique with its prestigious R&D 100 Award in 2011.

Further information can be found at www.mesocoat.com or by contacting Ghildyal at aghildyal@mesocoat.com.

REFERENCE
1. Canter, N. (2011), “Examining Corrosion and Oxidation at the Nanoscale,” TLT, 67 (4), pp. 10–11.

Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat items can be sent to him at neilcanter@comcast.net.