Wednesday, 29 October 2014

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].

control                    0.2
coupled to zinc       0.0
coupled to copper   5.3

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]. 

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.

Wednesday, 22 October 2014

Natural Finish Metallic Coatings – Attractive but not Architectural

Variations in Surface Appearance Caused by Viewing Angle

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.

Wednesday, 15 October 2014

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

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

Wednesday, 1 October 2014

Residential Steel Roofing Installation Considerations

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.