Monel: Characteristics, Uses and Problems

REFERENCES:

Margot Gayle, David Look, John Waite. Metals in America’s Historic Buildings. Washington, DC: National Park Service, 1995.

L. William Zahner. Architectural Metals. New York: John Wiley & Sons, 1995.

This document includes general information on the characteristics and common uses of Monel and identifies typical problems associated with this material along with common causes of its deterioration.

INTRODUCTION

Monel is a group of nickel alloys, primarily composed of nickel (up to 67%) and copper, with small amounts of iron, manganese, carbon, and silicon.

Characteristics of Monel:

• discovered due to the efforts of Robert Crooks Stanley, who worked for the International Nickel Company (INCO) in 1905
• named in honor of the president of the company, Ambrose Monell; the last “L” was dropped for trademark purposes
• stronger than steel
• malleable
• resistant to corrosion
• low coefficient of thermal expansion
• highly resistant to alkalis
• improved sanitation
• fairly inexpensive
• can be welded, brazed and soldered
• in the 1920s and 30s, Monel was available in both hot-rolled and cold-rolled sheets; today it is available in plates, rod, bar, tubing, and more rarely in castings
• typical finishes included brightly polished, “hand-forged” black, and two-toned
• in outdoor applications, Monel develops a patina ranging from light gray-green to medium brown

TYPICAL USES

• popular between 1909 and World War II when nickel was diverted to war use and lower priced stainless steel superseded Monel
• available in sheet form for architectural applications
• installed as a sheet roofing membrane in 1908
• in the late 1920s, Monel was began to be used for grocery coolers, countertops, sinks, laundry and food preparation appliances, roofing and flashing
• other uses for sheet and plate Monel were ductwork, flashing, gutters and downspouts, mail chutes, laundry chutes, elevator fittings, lighting fixtures, and skylights
• Monel castings were also popular and included grilles, rosettes, plaques, handrail fittings, molding, pilasters, mullions, and door jambs
• Monel forgings were used for hardware
• Monel bar and rod stock were used for window screens, gates, public directory boards, railings, and divider strips in terrazzo floors
• other common applications for Monel included tie wire for securing lath in plaster walls and suspended ceilings, fasteners for tile roofs and anchors for stone cladding
• Monel began to be displaced by stainless steel in the mid-20th century as stainless steel could produce the same forms at a lower cost (due to use of less nickel)
• a modified, less expensive use of Monel included laminating a thin sheet of Monel to an inexpensive backing material; two examples include Monel-clad steel and Monel-laminated plywood

NATURAL OR INHERENT PROBLEMS

• Surface discoloration: Can occur from exposure to atmospheric conditions
• Pitting: Can occur if exposed to stagnant salt water
• Corrosion:
  • nitric oxides and sulfur dioxides, combined with water, are very corrosive to Monel.
  • nitric and nitrous acids can be very corrosive to Monel at room temperature
  • hypochlorites are severely corrosive to Monel if not diluted
  • Acid and alkaline oxidizing salts, ferric chloride, ferric sulfate, cupric chloride, stannic chloride, mercuric chloride and silver nitrate are all corrosive to Monel.
  • resistance to sulfurous acid varies depending on climatic conditions
  • organic acids (acetic and fatty acids) have little to no effect on Monel
• Stress corrosion cracking: Exposure to aerated hydrofluoric acid in moist conditions can cause this to occur
• Galvanic corrosion: Metals, such as aluminum, zinc and iron will corrode when in contact with Monel AND exposed to severe weather conditions; therefore, use of these metals as fasteners for Monel should be avoided