Wet Storage Staining of Galvanized and Galvannealed Steel Sheet

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Most cold formed steel building products, whether painted or unpainted, are manufactured from a sheet steel material that has some form of metallic coating applied. This metallic coating can be zinc (galvanized), zinc-iron alloy (galvanneal) or a 55% aluminum-zinc alloy (GalvalumeTM). The metallic coating is available in a range of thicknesses to provide the degree of corrosion protection and service life required. One of the concerns expressed by installers relates to the presence of wet storage staining on the products, how this staining impacts the long term performance, and what can be done to remove it. The purpose of this blog post is to address some of these issues, allay some fears, and give guidance on proper storage techniques.

What is Wet Storage Stain?
“Wet storage stain” is a term used in the galvanizing industry to describe the zinc corrosion products that can form on a galvanized steel surface during storage. This staining is also referred to as “white rust”, which is the term generally applicable to all zinc corrosion products. Wet storage stain is voluminous, white, powdery, and bulky and is formed when closely packed galvanized articles are stored under damp and poorly ventilated conditions.

Often the white rusting appears to be quite heavy when, in fact, the amount of zinc corroded is small. This occurs because zinc hydroxide is somewhat voluminous and builds up in areas of wetness. Although wet storage stain can affect the appearance of the galvanized steel articles in some situations, it is generally not harmful in terms of the long-term corrosion performance.

Treatment of Galvanized Steel with Wet Storage Stain 
Galvanized sheet steel affected by wet storage stain can usually be cleaned, but generally cannot be restored to its original high luster appearance. The stain, depending on the severity, irreversibly alters the surface characteristics of the zinc to varying degrees. Nevertheless, there are treatments that are helpful in improving the appearance, depending on the severity of the problem.

Light white rusting:
This is characterized by the formation of a light film of white powdery residue. If left alone it may wash off in service with normal weathering. If it is deemed necessary to remove the white rust it can usually be done with a stiff bristle brush (nylon). If brushing alone is insufficient, rub or brush the surface with a mixture of mineral oil and sawdust. The mild abrasive action may remove the stain, although this treatment is not of much help for advanced wet storage stain.

Moderate white rusting:
If the stain is not too severe, it may be removed by washing with a 10% (by volume) acetic acid solution, followed immediately by a thorough rinsing with water to neutralize the surface. The removal can be assisted by the use of a stiff bristle brush (nylon). The sheets must be dry before restacking. This treatment may remove some of the metallic lustre, even in non-stained areas.

Severe white rusting:
The zinc hydroxide corrosion product will dissolve readily in weak acidic solutions. Ordinary household white vinegar has been found very effective and environmentally benign. Commercial products like CLRTM, widely advertised for scale and stain removal, can also be effective. Alternatively, a solution of 5% (by volume) of phosphoric acid in water, with a wetting agent added, can be brushed onto the sheets. In all cases proper safety precautions are necessary as well as approved disposal of cleaning liquids. After cleaning, the sheets must be immediately well-rinsed to neutralize the surface and then thoroughly dried. This treatment will remove some of the metallic luster, even in non- stained areas.

Dark grey or black staining:
If the stain has progressed to dark grey or black in colour, removal may not be possible. One method of restoring the protective value of the zinc coating, and improving the appearance of storage stain damaged sheets, is to apply a good, colour matched zinc-rich paint. The surface must be thoroughly brushed, rinsed and dried beforehand. After a period of time weathering will largely remove any difference in appearance between the zinc-rich paint and the original galvanized surface.
Note: Any field painting that may be required to cover wet staining is the responsibility of the buyer, not the deck supplier.

To learn more about metallic coatings, white rust and proper onsite storage of galvanized and galvannealed sheet steel, download our Fact Sheet 33: Wet Storage Staining of Galvanized and Galvannealed Steel Sheet

Cool Metal Roofing - Questionable Value in Cold Canadian Climates

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It’s a fact that buildings consume two thirds of all the electricity produced in North America and one third of all the energy produced in North America. While it is recognized that adding insulation under the roof surface can reduce cooling and heating costs, there is a diminishing return on the strategy of increasing insulation to conserve energy costs. This is where “cool roofing” can play a role in further reducing the energy consumed, and in minimizing the Heat Island effect created in the big urban cities. Cool roofing relies on sustainable, energy efficient, coated steel products, in a wide variety of finishes, colours, textures and roofing profiles. It conserves energy through its properties of reflectivity and emissivity.

Reflectivity

Reflectivity is the ability of the roof to reflect solar radiation back into the atmosphere. Its primary measure is solar reflectance - the proportion of the total solar radiation that is reflected back to the atmosphere. Any solar radiation that is not reflected is absorbed into the building envelope, requiring further energy to cool the building; or partially convected into the atmosphere increasing the ambient air temperature in the surrounding environment (Heat Island effect). This secondary measure is the Solar Reflectance Index, which takes into account the cooling effect of wind passing over the roof.

Emissivity

Emissivity is the ability of the roof to re-radiate absorbed solar infrared radiation back to the atmosphere. This takes place at all times, but mostly at night. Its measure is Infrared Emittance - the proportion of absorbed infrared solar radiation that is re-emitted back to the atmosphere. Most unpainted metallic coated steels have low emissivity (less that 0.12), while prepainted steels (regardless of colour) have emissivity values of around 0.9.

What is Important in Canada

In Canada, over the course of a full year, winter “heating load” conditions have a greater influence than the summer time “cooling load” because of the much longer and colder winters experienced in Canada. The degree of influence depends primarily on the geographic location, the insulation level in the roof assembly, and the cost of energy. For example, heating loads in Winnipeg, Manitoba are much higher than in Windsor, Ontario because of the greater number of heating degree-days in Winnipeg.

Calculations involving solar reflectance and emittance can be used to determine the energy savings attributable to a roof. Cool metal roofing can reduce energy costs associated with air conditioning. For cooling loads, it is advantageous to reflect as much solar radiation as possible, and to re-emit as much of the absorbed infrared radiation as possible. However, for energy savings associated with heating loads, absorption of solar infrared radiation is beneficial, and it is best to retain absorbed solar radiation, and not emit it back to the atmosphere. In other words, for Canadian climates, it is best to have roofing products with high reflectivity and low emissivity.

Conclusions 

Cool Metal roofs, with both high reflectivity and high emissivity, offer significant savings in reducing cooling loads. By definition, cool roofs reflect much of the solar radiation, and that which is absorbed is re-radiated by virtue of high emissivity. This is beneficial in predominantly warm climates like the southern US. The state of California has now recognized that different climatic conditions require different approaches. California has sixteen climate zones from north to south; however, they have exempted three of the sixteen climate zones (cold climates) from their Cool Roof (Title 24) regulatory requirements in recognition of the fact that colder climates are not well served by Cool Metal roofing.

In Canada, heating load predominates even in the warmest cities. Energy savings in Canada can be achieved by selecting materials with low infrared emissivity. Prepainted, asphaltic and membrane roofs have high emissivity and therefore re-radiate much of absorbed infrared solar radiation at night - a detriment in winter when absorption heat should be retained to reduce heating loads. A 55%Al-Zn roof combines the benefit of high reflectivity, which reduces summer cooling load, with the benefit of low emissivity, which reduces winter heating loads.

In Canada, the best choice for energy savings is a roof with high solar reflectance and low infrared emittance. However, it should be noted that if a building project stipulates Cool Roofing properties, CSSBI fabricator members have products that meet the reflectivity and emissivity requirements.

To see the calculations for each location in Canada and to learn more, download our Environmental Fact Sheet 2: Cool Metal Roofing - Questionable Value in Cold Canadian Climates

Applications of Non-Loadbearing Steel Studs

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Non-loadbearing steel studs (often called “drywall studs” are used throughout the Canadian construction industry for interior partition walls, bulkheads, drop ceilings and miscellaneous framing. These members are not “structural” since they are not designed as part of the structural load carrying system of the building; however, they are required to meet certain minimum standards related to material properties and dimensions. The CSSBI LSF Technical Bulletin Volume 7, Number 3: Specification of Non-Loadbearing Steel Studs defines the minimum thickness requirements.

Benefits of Non-Loadbearing Steel Framing

Low Cost:  Non-loadbearing steel stud framing has historically been less expensive than competing forms of construction.

Mass Produced: The basic building blocks of steel stud construction are cold formed channels manufactured from coated sheet steel on continuous roll forming mills. The coated steel is purchased by the manufacturer in the form of a master coil, which is subsequently slit into various widths as required by the geometry of the final products. The slit coil of flat steel is fed into one end of the roll forming mill and the cold formed channel member is cut to length as it emerges from the other end. (On some mills the sheet steel is cut to length before it enters the roll forming mill.) A typical mill can produce kilometers of straight, highly uniform, accurate product in one eight-hour shift.

Lightweight: Lightweight translates into cost savings in a variety of ways.

• Buildings not previously designed for an extra floor level may be candidates for expansion upwards. 
• Economical spread footings might be possible in marginal soil conditions as opposed to expensive deep foundations. 
• Low mass construction minimizes lateral bracing in seismic regions. 
• In panelized construction, low weight assemblies cost less to transport and erect. 
• In general, lightweight assemblies allow more economical structural framing because less material is required to support the self weight of the structure.

Download Technical Bulletin Volume 7, Number 4: Applications of Non-Loadbearing Steel Studs to learn more about the benefits of non-loadbearing steel framing, fire and acoustic ratings and more.

Sheet Steel Products and Pressure Treated Wood

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Many buildings will include wood members in applications such as sill plates, splash boards, strapping, purlins, door or window bucks, and posts. In some of these end-uses it is a requirement that the wood be chemically treated (pressure treated) to extend the service life.

Designers and builders need to be aware that changes in the available wood preservatives may impact the durability of any connected steel components or fasteners.

Effective January 1, 2004 the Environmental Protection Agency (EPA) banned the use of Chromated Copper Arsenate (CCA) as a preservative in treated lumber for residential construction. This was done in an effort to reduce the use of chromate and arsenic thereby mitigating the potential health and environmental problems. The wood preservative industry has been switching to alternative waterborne compounds including Sodium Borate (SBX), Alkaline Copper Quat (ACQ), Copper Azole (CBA-A and CA-B), and Ammoniacal Copper Zinc Arsenate (ACZA).

Unfortunately, research has indicated that ACQ, CBA-A, CA-B and ACZA, the new generation copper-based products, are more corrosive to galvanized steel than the former CCA. Since ACQ is becoming the predominant preservative in use, the discussions in this paper will refer to it exclusively.

Here are the recommendations for using sheet steel building products and pressure treated wood.

Sheet Steel Roong and Siding

In the construction of a wood frame building (i.e. pole barn), pressure treated wood is commonly used for the framing poles, roof purlins and skirt boards. The balance of the building wood components (i.e. trusses, strapping and girts) are from non-treated lumber.

It is common practice to attach the roong or siding to strapping or sheathing (i.e. plywood or OSB) that are in turn attached to the pressure treated wood structural members. Strapping and sheathing are generally not made from pressure treated wood and standard installation practices can be followed. The connection of the strapping or sheathing to the pressure treated framing requires added consideration.

It is recommended that all galvanized or painted sheet steel roofing or siding products be separated from ACQ pressure treated wood with Ice & Water Shield membrane or similar product. The use of roofing felts is not an adequate separator.

Lightweight Steel Framing

The following are options for cold-formed steel framing that should be considered:

• Isolate the steel and wood components
• Avoid use of pressure treated wood

Download Fact Sheet 18: Sheet Steel Products and Pressure Treated Wood to learn more.