Copper-Clad Stainless Steel Architectural Material ... · The copper clad stainless steel material was used for roofing, rain drainage, and flashing products in a large number of

Copper-Clad Stainless Steel Architectural Material Performance Report

Charles D. Tuffile, Ph.D.

Engineered Materials Solutions, Inc.

39 Perry Avenue

Attleboro, MA 02703

June 21, 2007

INTRODUCTION Copper clad stainless steel was introduced to the architectural products marketplace 40 years ago by Texas Instruments as an alternative material to copper architectural strip and sheet. The copper clad stainless steel material was used for roofing, rain drainage, and flashing products in a large number of installations in the late 1960’s and early 1970’s, many of which are still in existence today. Copper clad stainless steel not only is a means to reduce cost, minimize cost volatility, and improve supply availability, but the copper clad stainless steel material has also been proven to provide additional mechanical and thermal benefits due to the inherent properties of the copper clad stainless steel material, which result from the combination of copper and stainless steel. The additional benefits include high strength, low weight, good formability, low thermal expansivity, and excellent solderability. Since the surface layers of copper clad stainless steel are copper, they react as copper does and develop the same patina that makes copper a unique and beautiful building material. MATERIAL Copper clad stainless steel consists of a copper alloy (hereinafter referred to as Cu) conforming to ASTM B370 that is metallurgically bonded to both surfaces of Type 430 stainless steel (hereinafter referred to as 430SS). Copper clad stainless steel can be rolled to a number of gauges for a variety of architectural applications such as, but not limited to, roof pan, rain drainage, and flashing. The layer volume ratio of copper clad stainless steel is 10/80/10 Cu/430SS/Cu. Figure 1 shows a schematic cross section of copper clad stainless steel and Figure 2 shows a micrograph of a cross section of 0.016” copper clad stainless steel. The stainless steel core in copper clad stainless steel increases the strength of the clad material over Cu architectural strip. Table 1 shows typical mechanical and thermal properties for copper clad stainless steel, and some of the typical copper alloys that are used as copper architectural strip, which conform to ASTM B370. MECHANICAL PERFORMANCE The increased strength of copper clad stainless steel over Cu strip allows copper clad stainless steel to be used at thinner gauges than Cu strip while providing at least equivalent, and in many case improved, mechanical performance to thicker Cu strip. Figure 3 shows a theoretical plot of the load required to plastically yield various Cu architectural materials as a function of the strip thickness and temper. Additionally, the increased strength of copper clad stainless steel allows it to be used in applications or markets where Cu strip cannot satisfy current requirements due to the low strength of Cu strip; such as high wind load or heavy hail damage areas. Buckling and plastic flow are common modes of failure with Cu architectural materials. The primary variables that control buckling and plastic flow are the method of restraint, stiffness of the material, the moment of inertia of the column, and the length of the column. Given the same method of restraint and same column length, the lower moment of inertia and greater stiffness of Cu-clad 430SS over Cu alloys will result in less buckling of roofing and wall panels than Cu alloys. Figure 4 and 18 illustrate copper clad stainless steel fascia applications with no evidence of buckling, while Figure 5 shows contemporary Cu fascias that exhibit significant buckling. Underwriters Laboratories, UL 2218 test standard for “Impact Resistance of Roofing Materials” was used to measure the resistance to hail damage of copper clad stainless steel. The UL 2218 test standard adopted by the Standard Building Code was mandated by insurance companies, due to the large losses that have been absorbed in recent years from hail damage to asphalt shingled roofs. Roof coverings that meet this UL test standard are classified as either Class 1, 2, 3, or 4; with Class 4 being the most impact resistant. In certain areas of the country, particularly the Texas coastline, roofs with class 4 rated materials can qualify for an insurance premium discount. When 0.012” thick copper clad stainless steel was UL 2218 tested, it showed impact resistance that classifies as Class 4, the most aggressive impact resistant rating. The strength of the metallurgical bond between the Cu alloy and 430SS in copper clad stainless steel is such that the bonded materials cannot be mechanically separated using standard means, such as bend and cup tests. Copper clad stainless steel is capable of 180º, zero radius, bend with no evidence of separating of the bonded layers or cracking of the material. Figure 6 shows photographs of copper clad stainless steel after being subjected to 2T, ½T, and ¼T bends using the ASTM B 820 test. Also shown in Figure 6 is a specimen that was subjected to a ¼T bend, and was then folded flat 180º on itself. Biaxial tensile testing via the Erichsen cup method was used to measure the formability of Cu-clad 430SS and Cu. The methodology established by ASTM E 643 was used in the Erichsen cup test. Tested were 0.020” thick Cu-clad 430SS and 0.023” thick C12200-H00. The end point of the test was determined at the onset of visible necking. Three specimens of each material were tested. Figure 7 shows photographs of the formed cups after testing. The average cup height of the Cu-clad 430SS was measured as 0.381” with a 0.003” standard deviation, and that of the C12200-H00 was measured as 0.365” with a 0.005” standard deviation. Hence, the Cu-clad 430SS showed no evidence of separating of the bonded layers, and slightly better formability than C12200-H00.

THERMAL PERFORMANCE Table 1 shows the thermal properties of Cu-clad 430SS and other Cu alloys included in ASTM B370. The coefficient of thermal expansion (CTE) of Cu-clad 430SS is significantly less than that of the ASTM B370 Cu alloys. Figure 8 shows a dilatometer plot, measured per ASTM E 228, of Cu-clad 430SS and C11000 to highlight the difference in thermal expansion in a range of temperatures from -50 F to 250 F. Less thermal expansion in copper clad stainless steel versus Cu alloys provides less overall part expansion, which results in a number of benefits, that include: fewer expansion joints needed in products such as rain drainage products, and less potential for failed solder joints due to continual expansion and contraction of the soldered materials. The lower thermal conductivity of Cu-clad 430SS, in comparison to Cu alloys, results in more local retention of heat during soldering. Since the solder joint retains heat more than Cu alloys, the solder flows easier, which results in faster soldering and higher joint shear strength. CORROSION PERFORMANCE Erosion corrosion, the erosion of a metal that is caused, or accelerated, by the relative motion of concentrated amounts of water and the metal surface is a common cause of failure in fabricated architectural Cu components. Erosion corrosion is exacerbated when the concentrated water contains particulate matter and/or when acidic water is concentrated on a small area of Cu. The acidity of the water can deteriorate the Cu before a protective patina develops. The relatively high level of erosion corrosion in Cu alloys is due to the inherent softness of the alloys. The 430SS layer in the Cu-clad 430SS material provides a barrier to extensive erosion corrosion and limits this corrosion to the thickness of the Cu layer, leaving the structural integrity of the roof or rain drainage system intact. Figure 9 shows a photograph of a corner of a Cu-clad 430SS roof that experienced a high level of erosion corrosion. The bright 430SS layer is evident at the very corner where the Cu has eroded away. If this roof were designed with monolithic Cu, this same corner could have completely perforated resulting in structural failure at the joint. Laboratory and atmospheric exposure tests have shown that the surface of Cu-clad 430SS corrodes, as would monolithic Cu alloys. Hence, the layer volume ratio of copper clad stainless steel was designed as 10/80/10 (Cu/430SS/Cu) such that the clad material has sufficient Cu thickness to provide in excess of 50 years of service in a high-chloride coastal environment when starting with a 0.0160” thick clad material. Figure 10 is a plot of the penetration of copper due to corrosion versus the exposure time, which is based on field test exposure data in coastal, rural-humid, and rural-dry conditions. Figure 11 is a plot of the interpretation of the data from Figure 10 showing the thickness of copper that corroded versus the exposure time for coastal and rural-humid conditions. Figure 11 shows that the 0.0160” thick clad material, which has 0.0016” of Cu on each surface, is expected to provide in excess of 50 years of service in an aggressive, high chloride, coastal environment. The ASTM B 117 accelerated salt spray test was used to evaluate the edge corrosion resistance of Cu-clad 430SS, where the 430SS layer is exposed. The Cu surface corrodes, as would monolithic Cu; however, the edge is an area where galvanically induced corrosion could take place. Three 0.0216” thick coupons of Cu-clad 430SS measuring 2” x 5” were cut for salt spray testing along with monolithic Cu coupons. The edges of the coupons were ground to maximize the 430SS exposure. The samples were degreased prior to initiating the B 117 test. Photographs of the samples after 1000 hr of salt spray testing are shown in Figure 12. Cross sections of Cu-clad 430SS and C11000 specimens after 1000 hr of salt spray are shown in Figure 13. No appreciable red rust is apparent on the Cu-clad 430SS specimens after 1000 hr of salt spray exposure and there is no unusual loss of Cu thickness in the clad material. It is well known that the accelerated corrosion tests used to test the corrosion resistance of material that will be exposed to the elements does not provide any direct correlation to expected years of life. The best test of corrosion resistance is real-time exposure of the material to a variety of conditions. Cu-clad 430SS was used in roof and rain drainage installations in a number of locations throughout the United States in the late 1960’s and early 1970’s, and many of these installations remain in service today. Table 2 lists a number of these installations along with the installation location and architect and/or general contractor that designed the structure and incorporated copper clad stainless steel into the design. Figures 14-18 show photographs of the installations listed in Table 2 after roughly 40 years of service. Close inspection of the copper clad stainless steel installations shows no evidence of red rust or other defects that might be observed with the clad material, and the same patina formation as would be expected with Cu. Figure 19 and 20 show the chloride levels in air and rain, respectively, in which these installations exist.4 The chloride concentration in the atmosphere has a direct correlation to the potential for inducing corrosion of the stainless steel layer. Hence, the installations are located in areas with a variety of different atmospheric conditions, and chloride levels, yet no significant stainless steel corrosion is evident. CONCLUSIONS Cu-clad 430SS shows mechanical and thermal performance benefits over ASTM B370 Cu alloys. The benefits in mechanical performance include strength, resistance to buckling and plastic flow, impact resistance, and formability. The thermal performance benefits include low coefficient of thermal expansion and faster soldering with higher joint strength. The 430SS layer in Cu-clad 430SS has been shown to prohibit large-scale erosion corrosion, and thus prevent premature structural failure that could take place if the design were fabricated from Cu alloys.

Laboratory salt spray testing has shown that the exposed 430SS edge of the clad product is resistant to corrosion after 1000 hr of ASTM B117 salt spray testing. The salt spray test also showed that the corrosion behavior of the Cu in the clad material and in monolithic Cu architectural strip is similar. Examination of structures that incorporated copper clad stainless steel 40 years ago show that the clad material has weathered as would monolithic Cu, and has shown no evidence of red rust or other defects that might be related to the clad material versus monolithic Cu. REFERENCES 1. State of Texas, Office of the Secretary of State, Texas Register, Vol. 24, No. 24, June 11, 1999. 2. Copper Development Associations, Inc., Architecture Design Handbook. 3. Baboian, R., Haynes, G., and Sexton, P., “Atmospheric Corrosion of Laminar Composites Consisting of Copper on Stainless Steel,” Atmospheric Factors Affecting the Corrosion of Engineering Metals, ASTM STP 646, S.K. Coburn, Ed., American Society for Testing and Materials, 1978, pp. 185-203. 4. National Atmospheric Deposition Program (NRSP-3), 2007, NADP Program Office, Illinois State Water Survey, 2204 Griffith Dr., Champaign, IL 61820. Table 1: Typical mechanical and thermal properties for copper clad stainless steel, and some of the typical copper alloys that are used as copper architectural strip, which conform to ASTM B370.

Material

Temper

Tensile

Strength

Yield

Strength

Elongation

at Break

Density

Modulus of

Elasticity

CTE,

Linear

Thermal

Conductivity Cu-clad 430SS

Annealed 60 40 32 0.289 27 6.1 37

UNS C11000

O60 (soft) 32 10 45

UNS C11000

H00 (1/8 hard)

36 28 30

UNS C11000

H01 (1/4 hard)

38 30 25

UNS C11000

H02 (1/2 hard)

42 36 14

UNS C11000

H04 (hard) 50 45 12

0.323

17

9.4

226

UNS C12200

O60 (soft) 32 10 45

UNS C12200

H00 (1/8 hard)

36 28 30

UNS C12200

H01 (1/4 hard)

38 30 25

UNS C12200

H02 (1/2 hard)

42 36 14

UNS C12200

H04 (hard) 50 45 12

0.323

17

9.4

196

Table 2: Examples of installations of Cu-clad 430 SS made in the late 1960’s and early 1970’s that remain in service today.

Building

Location

Installation Date

Architect

Contractor

Gulf States Paper

Company

Tuscaloosa, AL

1969

Finch, Alexander, Barnes, Rothschild and Paschal

(Atlanta, GA)

Brice Building

Company

St. Timothy’s Church

Warwick, RI

1968 Robinson, Green and

Beretta (Providence, RI

-

First Lutheran Church

Greensboro, NC

1969

Little, Lee & Associates (Charlotte, NC)

Kirkpatrick & Associates

(Greensboro, NC)

Harris Bank

Hinsdale, IL

1969 John A. Mayes, Donald H.

Williams & Partners (Ellyn, IL)

R.H. Roberts

Construction Co. Reading Farmers Berklan Roofing &

Market Reading, PA 1970 - Sheet Metal Co. (West Reading, PS)

Figure 1: Schematic drawing of cross-section of Cu-clad stainless steel.

Figure 2: Micrographs of a cross section of 0.016” Cu-clad stainless steel. (a) 100X, (b) 500X magnification.

Figure 3: Theoretical load required to plastically yielding various Cu architectural materials as a function of the strip thickness.

Figure 4: No buckling is evident in the copper clad stainless steel fascia on the Whitpain Offices in Blue Bell, PA. Architect: Jack Levin, Philadelphia, PA.

Figure 5: Buckling of contemporary copper fascia on the (a) Goucher College Center in Towson, MD (Architect: Pietro Belluschi in association with Rogers, Taliaferro, and Lamb), and (b) Agudath Shalom Synagogue in Stamford, CT

(Architect: Davis, Brodz, and Associates).

Figure 6: Cross sectional micrographs of Cu-clad 430SS subjected to bend testing, per ASTM B 820, with good way bend radii of: (a) 2T, (b) ½T, (c) ¼T, and (d) ¼T then folded 180º flat on itself, and with bad way bend radii of: (e) 2T, (f) ½T, (g)

¼T, and (h) ¼T then folded 180º flat on itself.

Figure 6 (cont): Cross sectional micrographs of Cu-clad 430SS subjected to bend testing, per ASTM B 820, with good way bend radii of: (a) 2T, (b) ½T, (c) ¼T, and (d) ¼T then folded 180º flat on itself, and with bad way bend radii of: (e) 2T,

(f) ½T, (g) ¼T, and (h) ¼T then folded 180º flat on itself.

Figure 7: Erichsen cup tests for formability of Cu-clad 430SS and C12200-H00 per ASTM E643.

Figure 8: Dilatometer plots of the coefficient of thermal expansion of Cu-clad 430SS and C12200 from –50 ºF to 250 ºF.

Figure 9: Erosion corrosion of corner of roof. 430SS layer in the Cu-clad 430SS roof material prevented erosion through

the material thickness.

Figure 10: Penetration of copper due to corrosion versus the exposure time in coastal, rural-humid, and rural-dry conditions.

Figure 11: Corrosion of Copper under coastal and rural-humid conditions.

Figure 12: Cu-clad 430SS specimens after 1000 hr of ASTM B117 salt spray exposure.

Figure 13: Cross-sectional micrographs of (a) Cu-clad 430SS and (b) C11000 specimens after 1000 hr of ASTM B117 salt spray exposure.

Figure 14: The Westervelt Company headquarters building (formerly Gulf States Paper Company headquarters) in Tuscaloosa, AL. Copper clad stainless steel roof installed in 1969. Photograph taken January 24, 2007.

Figure 15: St. Timothy’s Church and Rectory in Warwick, RI. Copper clad stainless steel roof and fascia installed in 1968. Photograph taken January 26, 2007.

Figure 16: First Lutheran Church in Greensboro, NC. Copper clad stainless steel roof and fascia installed in 1969. Photograph taken January 18, 2007.

Figure 17: Harris Bank (formerly known as First National Bank) in Hinsdale, IL. Copper clad stainless steel roof and fascia installed in 1969. Photograph taken January 6, 2007.

Figure 18: The Reading Farmers Market in Reading, PA. Copper clad stainless steel mansard roof installed in 1970. Photograph taken March 1, 2007.

Figure 19: Chloride concentration in air.

Figure 20: Chloride concentration in rain.