Uncontrolled Corrosion of Base Metals

The underlying causes of corrosion are the same for all metals, all stemming from an electrochemical phenomena. But the ways in which corrosion manifests itself are characteristic of each particular metal.

Steel corrodes in the atmosphere with the formation of rust, which develops very rapidly on unprotected surfaces. In a clean atmosphere, aluminum slowly develops a white or silver grey patina.

Aluminum surfaces weather by a characteristic of pitting, and corrosion rates are often assessed by measuring the depth of the pits. The rate of pitting falls off after the first year or two, moving gradually to a standstill.

The strong, heat-treatable alloys of aluminum, with copper as one of the chief alloy elements, or certain fully heat-treated alloys with magnesium and silicon as major alloying elements, may manifest another type of attack, intercrystalline in nature, which may cause more pronounced loss of strength if allowed to continue. Such materials may require protection by conversion coatings, Anodizing, Nickel plating, or some other form of protection depending on the environment.

Distinguishing Corrosive attacks

Simple Chemical Attack - the corrosion of a metal by an Acid/Alkali is an obvious example of simple chemical attack. Simple chemical attack occurs when sulfides are in contact with steel or copper. Ordinarily, aluminum is not subject to such attack. A classic example of such chemical attack is sludge retaining rainwater in the bottom of guttering. In this case, a corrosive solution is held in constant contact with the metal, and rapid attack may follow. Another classic example of a simple chemical attack is the fading/discoloration of aluminum automobile trim as a result of automatic carwashes. Some automatic carwashes use an alkaline soap for the cleaning cycle. This alkaline solution combined with high temperatures creates an ideal condition for the aluminum to corrode.

Electrochemical Corrosion (Galvanic Attack) corrosion of a metal accelerated through contact with another metal in moist or wet conditions is known as bimetallic or electrolytic corrosion. This corrosion is due to the action of a simple voltaic cell. The presence of a conducting solution is essential to this phenomenon but the presence of dissimilar metals is not essential provided that a difference of potential exists. In addition to the nature of the two metals, the extent of galvanic attack depends upon many other factors.

Factors contributing to Galvanic Attack

• Concentration of the electrolyte, which determines its electrical resistance
• Nature of ions present in the electrolyte
  
• Polarization effects
  
• Effect of stable surface films on the metal
  
• Relative areas of anode and cathode
  
• The physical nature of the corrosion product
  
• Temperature variations

Each of these factors can influence the total resistance of the circuit. The following table is a compilation of solution potentials of metals and alloys with respect to a calomel electrode. It provides an initial guide to the possible effects of bi-metallic contact.

Galvanic Potential Corroded End (Anodic or Least Noble)

• Magnesium
• Magnesium Alloys
  
• Zinc
  
• Galvanized Steel or Galvanized Iron
  
• Aluminum Alloy 5052-H
  
• Aluminum Alloy 3004-S
  
• Aluminum Alloy 3003-S
  
• Aluminum Alloy 1100-S
  
• Aluminum Alloy 6053-T
  
• Alclad
  
• Cadmium
  
• Aluminum Alloy 2117-T
  
• Aluminum Alloy 2017-T
  
• Aluminum Alloy 2024-T
  
• Mild Steel
  
• Wrought Iron
  
• Cast Iron
  
• Nickel Cast Iron
  
• Lead-Tin Solders
  
• Lead
  
• Tin
  
• Brass
  
• Copper
  
• Bronze
  
• Copper-Nickel Alloys
  
• Monel
  
• Silver Solder
  
• Nickel
  
• Iconel
  
• Chromium Iron

18-8 Stainless Steel

• Type 304 (passive)
• Type 316 (passive)
• Hastelloy C
• Silver
• Graphite
• Gold

Protected End (Cathodic or Most Noble)

The corrosive nature of sea water and of coastal environments is partly due to the low electrical resistance of salt solution. Similarly, the bad effects of industrial atmospheres on metals arise largely from the sulphur laden compounds, sulphurous and sulfuric acids, which are largely formed as a result of burning coal, and which dissolve in the moisture in the air or in the rain as it falls, or in films of condensed water on the metal.

To summarize, the extent and type of moisture is an important factor in determining the severity of galvanic attack. For indoor service, where wetting is infrequent, galvanic corrosion normally is no problem. Outdoors, attack may be relatively rapid in sea coast and industrial environments, where contamination, hence conductivity, of rain and condensed moisture is high.

Considerations for material selection in corrosive environments:

• Select metals as close together in the galvanic series as possible.
• For the anodic protection of steel, metals above steel in the series should be selected, Or a protective coating (more noble than the mating alloy) should be applied.
  
• Avoid combinations having a smaller area of the more anodic metal than of the cathodic, to avoid excessive current density on the anodic areas.
• Insulate dissimilar metals wherever possible to minimize galvanic corrosion.
  

Aluminum Alloys

The corrosion-resistance of aluminum alloys is due to the presence on the surface of a very thin protective film of aluminum oxide which has strong self-healing properties when damaged. The oxide film begins to form immediately on the surface of the bare metal exposed to air and grows rapidly for several days, then slowly for a month, when it reaches a thickness of approximately 0.0000002”. Corrosion of aluminum can only occur when the oxide film is damaged or removed and conditions prevent its formation. Substances which may come in contact with aluminum can be divided into three groups:

Those substances which attack the oxide film.

These are most strong alkalis, mercurical compounds, and most strong acids.

Substances which cause localized breakdown of the oxide film (pitting).

And for which aluminum is suitable only under certain conditions, such as some natural fresh waters and aqueous solutions containing traces of mercury, copper, or other heavy metals.

Substances which do not attack the oxide film.

The majority of substances fall in this group, including many industrial chemicals. The majority of aluminum installations give perfectly satisfactory service, free from corrosion, and only in exceptional cases do problems occur.

Contributions to Corrosion of Materials vs. Application

• Wrong choice of alloy
• Exposure conditions
• A bimetallic joint which causes galvanic corrosion
• Crevices
• Unwise location of the aluminum assembly, resulting in deposition corrosion
• Contact with aggressive chemicals Among the heat-treatable alloys, the 6000 series has good resistance to industrial and marine atmospheres.With the exception of certain corrosive chemicals, no corrosion at all will occur if water is not present. Thus, an indoor installations that is not in actual contact with water ( no electrolyte present) or installations which are maintained in dry conditions, will not be conducive to corrosion.