Aluminum: Characteristics, Uses And Problems

CSI Division Division 5 - Metals Section Metal Materials Last Modified

Technical Procedures Disclaimer

Prior to inclusion in GSA&#;s library of procedures, documents are reviewed by one or more qualified preservation specialists for general consistency with the Secretary of Interior Standards for rehabilitating historic buildings as understood at the time the procedure is added to the library. All specifications require project-specific editing and professional judgement regarding the applicability of a procedure to a particular building, project or location. References to products and suppliers are to serve as a general guideline and do not constitute a federal endorsement or determination that a product or method is the best or most current alternative, remains available, or is compliant with current environmental regulations and safety standards. The library of procedures is intended to serve as a resource, not a substitute, for specification development by a qualified preservation professional.

NB supply professional and honest service.

Rewrite

We&#;ve reviewed these procedures for general consistency with federal standards for rehabilitating historic buildings and provide them only as a reference. Specifications should only be applied under the guidance of a qualified preservation professional who can assess the applicability of a procedure to a particular building, project or location. References to products and suppliers serve as general guidelines and do not constitute a federal endorsement nor a determination that a product or method is the best alternative or compliant with current environmental regulations and safety standards.

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

Introduction

Characteristics of Aluminum:

• Lightweight
• Corrosion-resistant
• Nonmagnetic
• Has a low melting point
• Has a moderately high coefficient of expansion
• Has a good thermal and electrical conductivity
• Malleable
• Very soft
• Ductile

Aluminum found in historic buildings may be finished in one of the following ways:

• Nonfinished: A bare aluminum surface.
  • Upon exposure to the air, bare aluminum develops a thin layer of natural oxide. This patina layer is thin, transparent, tough, and protects the aluminum from corrosion.
  • The texture of bare aluminum may be smooth, highly polished or brushed, or it may obtain its texture from casting, extruding, or machining.
  • Nonfinished aluminum is the most common type of finish found on historic buildings ( -), both outdoors and indoors.
• Anodized: An oxide coating applied by passing an electrical current through the aluminum.
  • This tough coating is approximately 0.05 to 1.5 mils thick and provides greater resistance to atmospheric corrosion.
  • Anodized aluminum surfaces appear off-white in color and have more of a smooth finish than ordinary aluminum.
  • The anodic coating may be transparent or integrally colored by adding pigments or dyes before it is sealed.
    • In the s, colored aluminum was achieved by adding dyes. Colors of red, blue and green often faded nonuniformly and appeared blotchy. Colors of gold, brown, grey and black, however, usually retained their original color.
    • Today colored coatings are produced by varying the alloy content, which results surface only during the anodizing process. Any working of the metal and any texturizing of the surface is applied to the aluminum before anodizing.

• Anodizing aluminum was invented in and began to be used for architectural elements in the s.

• Chemical conversion: A coating formed by chemical processes.
  • This type of coating is thinner and less abrasion resistant than anodic coatings. It is often used as a base coating before painting.
  • The final finish of a chemical conversion coating may appear clear or colored. Some colors include gold, gray, golden brown, green, or blue-green.
• Painted/lacquered:
  • Pigmented (paint) or clear (lacquered) types of organic coatings were used in the s on aluminum doors, frames, and radiator cabinets to create a wood grain finish.
  • Today paint is usually applied over chemical conversion finishes. During construction anodized surfaces are often given a clear coating for protection against alkaline building products.
  • Aluminum siding with a baked-on paint finish came on the market in the s.
• Plated: The process of electrodepositing a metal onto the aluminum surface.
  • The most common metals used for plating are chromium and nickel. To achieve a smoother finish, copper may be used as an intermediate layer. Tin, silver or gold may also be used.
  • Plated aluminum is most commonly used for features that may be subjected to heavy abrasion, such as stair railings.

• Porcelain enameled: A baked-on ceramic coating applied in the factory.
  • It is hard and impervious to soils, many acids and alkalies. It is available in many colors and surface textures.
  • Seldom found in today&#;s historic buildings, its use as an exterior wall cladding beginning in the s will make it an historic material in the not so distant future.
• Laminated: Fabricated by bonding wood, cloth, plastic, etc. onto the aluminum. These types of finishes were introduced in the s.

Typical Uses

Typical historical uses for aluminum in the late 19th century included:

• Stairs
• Elevators
• Grilles

Typical uses for aluminum in the early 20th century included:

• Decorative detailing
• Roofing, wall panels, and spandrels (since it could be rolled into sheets)
• Window mullions and frames, storefront surrounds, doors, and door trims (as it could be extruded into lengths of specialized profiles or cross sections)

Problems and deterioration

Problems may be classified into two broad categories:

• Natural or inherent problems based on the characteristics of the material and the conditions of the exposure, and
• Vandalism and human- induced problems.

Although there is some overlap between the two categories, the inherent material deterioration problems generally occur gradually over long periods of time, at predictable rates and require appropriate routine or preventive maintenance to control. Conversely, many human induced problems, (especially vandalism), are random in occurrence; can produce catastrophic results; are difficult to prevent, and require emergency action to mitigate. Some human induced problems, however, are predictable and occur routinely.

Natural or Inherent Problems

Natural Corrosion:

• Upon exposure to the air, aluminum protects itself by developing a layer of white aluminum oxide which covers the exposed surface. This layer is thin, transparent, tough, and to a great extent protective.
• Heavy deposits of soiling occur when the aluminum surface has been neglected and not cleaned regularly.

Chemical Corrosion:

• Aluminum has good corrosion resistance to: Atmospheric gases, moisture and soil.
  Aluminum has poor corrosion resistance to:
• Alkalis, hydrochloric acid, lead-based paints, some wood preservatives, and chlorides.
• Aluminum may also corrode when in contact with wet lime mortar, Portland cement, plaster, or concrete before they are fully cured, damp, porous brickwork and stonework. To protect aluminum against contact with masonry, apply a coating of bituminous paint, followed by 2 coats of aluminum metal and masonry paint.
• Acids from unseasoned wood, damp oak, cedar, and redwood may also attack aluminum.
  • Corrosion will result from direct contact between wet wood and aluminum.
  • Water draining off a roof of unweathered wood shingles will also corrode aluminum.
• Corrosion may be accelerated on an aluminum roof where condensation develops on the underside of the roof, much like a terne- or tin plated roof. If standing water is acidic, corrosion cells will develop on the aluminum.
• Aluminum may be protected from corrosion by applying a paint or other coating as recommended by the Paint Manufacturer&#;s Association.

Galvanic (Electrochemical) Corrosion:

• Galvanic action will occur, causing the aluminum to corrode, if the aluminum comes in contact with other metals such as tin, iron and steel (if they are not painted), and especially copper.
• Aluminum is compatible with zinc, cadmium, lead, galvanized steel, monel, magnesium, and usually nonmagnetic stainless steel. Nonmagnetic stainless steel is sometimes corrosive to aluminum when the two metals come into contact in industrial environments.
• Protect aluminum against galvanic corrosion by electrically insulating it with a coating of paint or mastics.

Vandalism or Human-Induced Problems

Mechanical or physical deterioration:

• Erosion: Aluminum features are extremely vulnerable to erosion because this metal is so soft. When exposed to abrasive agents, erosion of aluminum can be a critical
  problem.
• Fatigue: Aluminum has a high coefficient of thermal expansion. Fatigue is one of the most common failures resulting from the stresses associated with expansion and contraction.
  • The lack of an adequate number of transverse joints or welts in a length of sheet aluminum between bays will result in cracking of the sheets.
  • Improperly sized bays (space between vertical seams) and an inadequate number of fasteners can also cause aluminum roofing to bow, buckle and eventually crack. Using aluminum sheets which are not rigid enough to resist this movement will exacerbate this problem.
  • Inadequate support from the underside, such as spaced rather than tight sheathing boards, will also result in buckling and sagging of the sheet metal, ultimately causing the metal to crack and tear.
• Human Error: The alloy specified for a job may not be the best choice for the function and location, or the material used in the fabrication is not the alloy specified. These mistakes can cause exfoliation of the aluminum, where the aluminum alloy begins to flake off in layers, similar to rusting wrought iron. The corrosion material lifts out from the surface as if the metal had exploded.
• A number of aluminum alloys have been developed to improve certain properties. These include different alloys for exposed outdoor locations, unexposed outdoor locations, and indoor locations:
  • Nonheat-treatable alloys, which include 1-1/4% manganese and 2 to 7% magnesium, are of relatively high strength and are used for cladding and also for corrugated roofing.
  • Heat-treatable alloys contain varying proportions of aluminum, magnesium, silicon, and sometimes copper. These have high strength and are, therefore, used for fasteners and for light structural members.
  • Aluminum alloys used for casting usually contain silicon, silicon and copper, or silicon and magnesium.

Anodizing is a method for changing the surface chemistry of metals and other substrates. It protects against corrosion, enhances aesthetic qualities, resists scratching and is one of the most durable surface finishes available. Anodizing can be done on a range of materials, but today let&#;s look at aluminum. These 8 questions will help to show why anodizing is a clever surface treatment that&#;s both practical and beautiful.

To prepare aluminum for anodizing, the surface is first thoroughly cleaned and rinsed, and then placed into a bath of some electrolytic solution like sulfuric acid. An electrolyte is an electrically conductive solution with lots of positive and negative ions that it wants to swap.

A positive electric charge is applied to the aluminum, making it the &#;anode&#;, while a negative charge is applied to plates suspended in the electrolyte. The electric current in this circuit causes positive ions to be attracted to the negative plates, and negative ions to flock to the positive anode, the piece of aluminum.

 The electrochemical reaction causes pores to form on the surface of the aluminum as excess positive ions escape. These pores form a geometrically regular pattern and begin to erode down into the substrate. The aluminum at the surface combines with the negatively charged O2 ions to create aluminum oxide. This is called a barrier layer, a defense against further chemical reactions at those spots.

Are you interested in learning more about aluminium protective solution? Contact us today to secure an expert consultation!

The longer the current is applied the greater the penetration of these columns. For typical non-hard coatings, the depth can be up to 10 microns. Once this level is reached, and if no color is needed, the process is stopped and the surface can be sealed simply by rinsing in water. That will leave you with a hard, natural aluminum oxide coating, able to withstand chemical attack and very scratch resistant. Aluminum oxide is rated 9 out of 10 on the Mohs hardness scale, meaning second only to diamond.

Hard anodizing, sometimes called Type III, offers greater corrosion protection and resistance to wear in extreme environments or with moving mechanical parts subject to a lot of friction. This is produced by continuing the electrical current until the depth of the pores exceeds 10 microns, all the way to 25 microns or even more. This takes more time and is more expensive but produces a superior result.

Although aluminum doesn&#;t rust, it can deteriorate in the presence of oxygen, which is called oxidation. What is oxidation? It simply means to react with oxygen. And oxygen is very reactive, readily forming compounds with most other elements. When aluminum is exposed to the atmosphere it quickly forms a layer of aluminum oxide on the surface, and this layer provides a degree of protection against further corrosion.

The pigment fills all the empty pores up to the surface, where it&#;s then permanently sealed off. That&#;s why anodized colors are so durable &#; they can&#;t be scratched off from the surface because in fact the colors are deep down and can only be removed by grinding away the substrate.

After coloring, anodized aluminum has a characteristic &#;metallic&#; look. This is caused by two factors. One, because of the uniform electro-chemical etching, a rough surface is left behind. The deeper the pores, the rougher the surface will be but the colors will also be that much more durable.

Secondly, light striking the surface partly interacts with the colorant and partly with the uncolored metal at the top.

Anodizing requires that a part is immersed in a series of chemical baths. Holding a part in position requires that it be mounted on a hanger of some kind to keep it from falling to the bottom of the tank. Wherever the holding fixture touches the part, that area will be blocked and the anodizing chemicals won&#;t work properly. That&#;s why it&#;s smart to design a place on your part which can be used for holding but which won&#;t be adversely affected cosmetically.

When you contact us for a free quotation and project review, we&#;ll be able to offer advice on the many different finishing services that we offer for rapid protoypes and low-volume manufacturing. Our specialists will help you to find the solution that fits your budget, time to market and desired results. Let&#;s get started today!

*Star Rapid is grateful to Engineerguy for the use of some illustrations under Creative Commons License Share Alike License 3.0.  

For more information, please visit Laser Cutting Protection Film Supplier.