Galvanic Compatibility of Galvanized Steel and Aluminum

In many practical construction applications the contact of dissimilar materials is sometimes unavoidable. When dissimilar metals are in contact with one another in the right medium the condition is called Galvanic Coupling. The effects of galvanic coupling depend on how different the electrochemical properties of the metals are. The following Technical Bulletin describes the compatibility of Galvanized steel and Aluminum, two materials commonly found together in the construction of lightgauge steel framed homes. Dr. X.G. Zhang is a Corrosion Scientist for Cominco Ltd., and is author of Corrosion and Electrochemistry of Zinc.

Galvanic Compatibility of Galvanized Steel and Aluminum
By X.G. Zhang, Cominco Ltd.

Zinc and aluminum are galvanically compatible materials in atmospheric environments. That is, when these two metals are in direct contact there will be very little galvanic corrosion of either metal resulting from the coupling.

As shown in the Table 1 below, the amount of corrosion of both zinc and aluminum when coupled to each other is close to that of the controls, indicating that there is very little galvanic corrosion. This is in contrast to the coupling with copper for which the amounts of corrosion on both zinc and aluminum are greatly increased due to the galvanic action. The reason for the low galvanic action between zinc and aluminum is primarily due to a lower position in the electromotive force series of aluminum relative to zinc and the formation of an inert passive film on the surface of aluminum.

Table 1
Galvanic corrosion rates of zinc and aluminum tested for one year in an urban atmospheric environment, in m/y [1].

Aluminum
control                    0.2
coupled to zinc       0.0
coupled to copper   5.3

Zinc
control                         1.2
coupled to aluminum  1.1
coupled to copper       2.0

Test in a wire-on-bolt assembly

Because of their galvanic compatibility, zinc and aluminum can be used together in atmospheric environments without significant galvanic corrosion problems. The situation is even better when the metals are painted. Since paint is generally not conductive, it prevents the electrical and/or electrolyic contact between the two metals which is required for galvanic action. Therefore, painted aluminum and galvanized steel can be used in direct contact without causing galvanic corrosion problems as, for example, shown by the sketch above in the case of a galvanized steel fascia in contact with a painted aluminum eavestrough. Some galvanic action may occur at places where the two painted metal products are joined by metallic screw fasteners or nuts and bolts. At these places the amount of galvanic corrosion should be close to the values indicated in the table and the extent of galvanic action is limited to within a few millimetres of the contact line [2]. 

References: 

1. V. Kucera and E. Mattsson, “Atmospheric Corrosion of Bimetallic Structures”, in Atmospheric Corrosion, W.H. Ailor (ed.), pp.561-574, John Wiley & Sons, New York, 1982. 

2. X.G. Zhang, “Galvanic Protection Distance of Zinc Coated Steels Under Various Environmental 

Conditions”, Corrosion’98, Paper No. 747, NACE, 1998. 

Click to download Technical Bulletin Volume 2, Number 2: Galvanic Compatibility of Galvanized Steel and Aluminum

Natural Finish Metallic Coatings – Attractive but not Architectural

Variations in Surface Appearance Caused by Viewing Angle

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Architects and Specification Writers are increasingly selecting unpainted metallic coated steels for architectural roofing and cladding applications on building exteriors where they want a “Silver” metallic finish. This is occurring more frequently, and even on “prestige” type projects. The Canadian Sheet Steel Building Institute whose fabricator members manufacture a wide variety of building panel profiles for roofing and cladding applications, are being asked to supply unpainted (natural finish) galvanized or resin coated 55% Aluminum-Zinc coated steel for these architecturally exposed end uses. Oftentimes, these materials are specified because the designer finds the natural finish of these products very appealing and sometimes because of material cost savings opportunities.

This blog post is to provide guidance in material selection and provide information on the Architectural Metallic Finishes that are available for highly visible steep slope roofing and cladding applications.

The recommended product for these applications is prepainted steel available in a wide variety of metallic finishes that are consistent in colour, gloss, reflectivity and overall appearance from panel to panel, regardless of the building elevation. A selection of metallic colours is shown below. It is important to note that the actual colours and finish may vary from these printed samples. If an exact colour match is required, contact a CSSBI Fabricator Member.

Prepaint Coatings
Prepaint coatings are applied to steel by a continuous coil coating process under strict quality control conditions. These Architectural (exposed quality) finishes are offered in a variety of metallic colours including, for example, Bright Silver. Depending on the end use requirements, metallic colours are available with either fluorocarbon (Kynar) or polyurethane paint systems to match silver, copper, bronze, aluminum, zinc or other metallic finishes. The prepaint systems are designed to match a colour standard and quality control measures during the paint process provide consistency across the width of the coil, along its length and from coil to coil. Each new batch of paint is also produced to the same colour standard to minimize batch to batch variation. Even with these quality control procedures in place, caution should still be exercised if more than one production order must be used for the same building. For recommendations, see Appendix A2 of CSSBI 20M-99 “Standard for Sheet Steel Cladding for Architectural, Industrial and Commercial Building Applications”.

Architectural prepaint systems also come with an exterior weathering performance specification that specifies a maximum colour change, chalking and film integrity as long as 35 years.

Architectural prepaint systems have proven and predictable weathering performance. They provide a consistent colour match to metallic finishes and should be the product of choice for applications that require uniform appearance.

Natural Hot Dip Metallic Coatings
The most common hot dip coatings used for building products like roofing and cladding are zinc and 55% aluminum-zinc alloy coatings. Both products are produced by the continuous hot dip galvanizing process. The quality control measures provide for good coating adhesion necessary for forming into profiles, and coating weight (thickness) to meet the appropriate ASTM coating designation for long service life.

Although there are manufacturing process metrics to control surface appearance, there is always normal variation in spangle size from coil to coil and within a coil. The natural metallic finish can therefore vary depending on steel substrate thickness and chemistry, pot chemistry and temperature, and other operating parameters as well as the roofing or cladding panel orientation (see image at the top). Unpainted hot dip coated steels are also passivated with a very thin inorganic or organic system to provide protection against storage stain. In spite of this, the weathered appearance of the metallic coating can become nonuniform over time and would not be consistent with an architectural finish.

In summary, unpainted natural finish hot dip metallic coatings are attractive and are used for a variety of commercial, industrial, and agricultural buildings for roofing and cladding. However, they are not considered to have an exposed architectural finish. If a uniform visual appearance is required over the long term, prepainted steel should be specified. A wide selection of prepainted steel having metallic finishes are currently available and new or unique metallic colours can be quickly developed to suit high profile projects.

Click to download Fact Sheet 24: Natural Finish Metallic Coatings – Attractive but not Architectural

Steel Cladding – A Farmer’s MVP (Most Valuable Product)

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Steel’s versatility and durability have made it an ideal building material for various construction projects for the past 150 years. Over that time, steel has earned a well deserved reputation for economy and proven performances with long life cycles. Combine these benefits with steel’s ability to be recycled and engineered for retrofits, and steel cladding undoubtedly will become the number one choice of building materials across all industries.

The Canadian Sheet Steel Building Institute commissioned a non-biased third party, Strategic
Research Associates, to examine the state of the Canadian farm. Specifically, the study examined farmers’ steel cladding purchasing habits and steel cladding usage over the past 10 years. The study queried 471 farms across Canada with 43 farms in British Columbia; 96 in Alberta; 96 in Saskatchewan/Manitoba (combined); 97 in Ontario; 96 in Quebec; and 43 in the Atlantic Provinces. The results are within ± 4.5 percentage points for complete representation of all Canadian farms and are as follows:

The Changing Canadian Farm

Since 1996, there has been a significant shift in farm type across the nation. Livestock farms have dropped by almost 20%, and the balance has shifted to a greater number of mixed crops (up to 28%) and cash crops (up to 39%) respectively. The study found that overall, there are fewer farms across Canada; however, the farms that do exist are considerably larger.

The Purchase of Steel Cladding

Strategic Research Associates note that a 10-year period is too long to adequately explain strong trends in increases of steel cladding purchases. However, the results are quite interesting as a whole as well as regionally. In 1996, only 47% of Canadian farmers said they had purchased cladding in the last 10 years. By 2006, that percentage of farmers grew to 79%.

Regionally, it was found that the nation’s three major markets are consistent with the overall average as shown in the previous graph. The study noted that significant growth occurred in the Quebec market. This is explained through the expansion of swine and dairy operations and the replacement of existing building stocks.

Steel, the Right Choice

While the study showed moderate increases in steel cladding purchases on the farm, the question remained on ‘how’ popular it was compared to other types of exterior cladding such as wood, vinyl, or aluminium. Results concluded that steel cladding is the top seller and gaining market share at the
expense of vinyl and aluminium. In 2006, 88% of Canadian farmers preferred steel cladding for their farm structures, which is up from 1996 by 9%. (See Figure 3.)

The types of farm buildings steel cladding is used for is consistent with 1996 numbers:

• 80% use it for machinery sheds
• 69% for storage buildings
• 65% for barns 
• 26% for houses 

Other factors to note about choosing steel:

• When given a choice, Canadian farmers choose Canadian steel 
• 90% of Canadian farmers are either satisfied or very satisfied with their steel cladding

Click to download Fact Sheet 26: Steel Cladding – A Farmer’s MVP (Most Valuable Product) to learn more.

Residential Steel Roofing Installation Considerations

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One of the most common questions asked by homeowners about the installation of their steel roof is whether an underlayment is needed. The answer to this question is “yes” in most situations. The underlayment plays a critical role in controlling the migration of condensation that might develop on the underside of the steel sheet thereby preventing accumulated water entering the building resulting in costly damage.

Underlayment is a general term used to describe a membrane installed between the steel sheets and the sheathing (plywood or OSB) or roof framing. There are a variety of materials used to manufacture underlayments with the most common being an asphalt impregnated organic fibre (roofing felts). The minimum weight of roofing felt should be equivalent to a #30 (30 pound). There are also premium synthetic products available that provide improved performance where required or desired.

The underlayment also provides a valuable second layer of protection against water getting into your home whether from wind-driven rain or from any condensation that may still occur on the back of the steel sheets. The only situation where an underlayment may not be necessary is an un-heated building (e.g. garage or storage shed) that does not contain any source of moisture (e.g. livestock or humid materials) or materials that could be damaged from possible moisture.

Click to download Fact Sheet 35: Residential Steel Roofing Installation Considerations to learn more about underlayment, strapping and building code requirements for roof installations.

Two new Design in Cold Formed Steel Seminars Scheduled

Our sister organization, the Cold-Formed Steel Engineers Institute Canada is hosting a Design in Cold Formed Steel Seminar. Below are the details.

Design in Cold Formed Steel: Using the North American Specification for the Design of Cold-Formed Steel Structural Members CSA Standard S136-12

When & Where

Tuesday, November 25, 2014 in Fredericton, NB

Wednesday, November 26, 2014 in Halifax, NS

Registration:

CFSEI Members - $225

Non-Members - $275

Each registrant will also receive a comprehensive set of lecture notes full of explanatory material and worked examples.

Registration is limited to 50 people on a first-come first-serve basis.

About the Seminar

The primary objective of this seminar is to make the designer conversant with the latest edition of CSA Standard S136-12 (North American Specification for the Design of Cold-Formed Steel Structural Members). This is a harmonized document between Canada, the US and Mexico, and supersedes the 2007 edition (including Supplement 2010). The Specification was developed through a joint effort of the American Iron and Steel Institute’s (AISI) Committee on Specifications and the Canadian Standards Association’s S136 Technical Committee. In comparison to the 2007 edition of S136 (including Supplement 2010), a number of significant changes have been incorporated into the North American Specification, in part due to the harmonization process and in part due to latest research developments. 

Topics
The intent is to bring the participant up-to-date with the current design provisions contained in the new North American Specification for the Design of Cold-Formed Steel Structural Members (S136-12), highlighting significant changes from the 2007 edition of S136. As well, numerous illustrative examples will be presented.

• Introduction 
• Materials
• General Design Considerations
• Elements in Compression
• Members in Tension
• Members in Bending
• Members in Compression
• Combined Bending and Compression
• Connections
• Member Bracing
• Testing and Fatigue
• Direct Strength Method

Also, the latest Editions of the AISI North American Design Standards for Cold-Formed Steel Framing will be reviewed since these design standards are referenced by CSA S136 for use in Canada.

Registrants are encouraged to bring a copy of the S136-12 Standard to the seminar. If necessary, this can be purchased from CSA by telephone [416-747-4044, or 800-463-6727], E-mail [sales@csa.ca] or by visiting their web site at www.csa.ca.

Schedule
8:00 am     Registration and coffee
8:30 am     Seminar begins
10:15 am   Break
12:00 pm   Lunch provided
12:45 pm   Seminar resumes
2:30 pm     Break
4:30 pm     Seminar concludes

Who Should Attend

• Anyone involved in the design of cold formed steel structural members. This seminar will provide a quick and effective means of learning about the 2012 edition of CSA S136 (North American Specification for the Design of Cold-Formed Steel Structural Members).
• Anyone who would like the opportunity to have questions answered concerning all aspects of cold formed steel design.

Speakers

Dr. R.M. Schuster, P.Eng.
University of Waterloo
Waterloo, Ontario

Dr. S. R. Fox, P.Eng.
General Manager
Canadian Sheet Steel Building Institute
Cambridge, Ontario

CSSBI Colleague Thanked for Contribution to the CEE Structures Lab at the University of Waterloo

Dr. Reinhold Schuster, respected colleague of the Canadian Sheet Steel Building Institute was honoured last week at a ceremony at the University of Waterloo Department of Civil and Environmental Engineering (CEE) Structures Laboratory. Dr. Schuster was instrumental in securing the donation of a new 2500 kN portal frame for the CEE Structure Laboratory.

Click here to read the thank you letter from the University to Dr. Schuster.

Application of UL Fire Resistance Ratings in Canada

The National Building Code of Canada, Part 3 on Fire Protection, Occupancy Safety and Accessibility, requires the fire resistance ratings for assemblies to be determined on the basis of tests conducted in accordance with CAN/ULC-S101 “Fire Endurance Tests of Building Construction and Materials”. S101 is a Canadian test standard used by agencies like Underwriters’ Laboratories of Canada (ULC) to conduct fire testing of building components. ULC listings have been used by Canadian design professionals for many years to select fire rated building assemblies, but there is another source for listings that significantly increases the available options: these are the Underwriters Laboratories Inc. (UL) tested assemblies.

It was always possible to use the UL listed assemblies in Canada, but questions were raised about the equivalence of the UL tests to the requirements of CAN/ULC-S101, and in particular the impact of UL loads calculated using Allowable Strength Design instead of Limit States Design as required in Canada. To address this difference in design approach, the UL designs included a “load restricted factor” (LRF) to reduce the design load for Canadian applications.

Working with UL and ULC, representatives of the Steel Framing Alliance, Canadian Steel Construction Council and the American Iron and Steel Institute were successful in getting adopted a LRF of unity for load-bearing cold-formed steel wall and floor assemblies listed in UL’s directory. Having no load restriction is possible because the calculation of the member resistance in Canada and the U.S. is based on the same standard: CSA S136-07 or ANSI/AISI S100-07, “North American Specification for the Design of Cold-Formed Steel Structural Members”.

Prior to this development, only assemblies that were ULC rated were readily accepted, and because very few tested assemblies were listed in their directory, cold-formed steel faced a significant barrier to entry into the mid-rise construction segment. With the removal of any load restriction, about 30 UL fire-rated load-bearing wall assemblies can now be used in the Canadian market. This LRF can only be applied to load-bearing wall assemblies tested with laterally braced steel studs which account for the majority of UL listed assemblies. In comparison, a more conservative factor of 0.82 and 0.65 must be applied to wood framed walls and floors respectively.

For More Information from UL
For more detailed information, please refer to UL’s website (www.ul.com) and open “BXUV7.GuideInfo” (on the bottom of their homepage, click on “Certifications”, then entre BXUV7 in the “UL Category Code” box and click search, then click on the “link to file” BXUV7.GuideInfo). For more specific help, contact the Standards and Codes Consultation Services staff at ULC through their website at www.ulc.ca.

Click to download Technical Bulletin Volume 1 Number 3: Application of UL Fire Resistance Ratings in Canada