Understanding How Metals Corrode Can Help Build Better

                         Structures



Except for the "precious" metals, such as gold, metals in the refined form are

inherently unstable. This instability is what drives the process of corrosion, and it

results from the fact that a refined metal is continually trying to revert to its

natural state (the mineral). Some metals do this faster than others.



The Galvanic Series ranks corrosion tendencies in specific environments. The

Galvanic Series for seawater is a much-used ranking because it’s a good,

general approximation of how metals behave. See Table 1.



How surface reactions alter a metal’s corrosion resistance can be seen in the

example of four common construction metals - aluminum, lead, copper and iron.

Aluminum ranks as a very active, or corrosion-prone, element in both the AMF

Series and the Galvanic Series for seawater, yet it is prized for its low

maintenance and slow corrosion rate. This is because aluminum forms a tightly

adhering surface film of aluminum oxide when exposed to the air. Under most

atmospheric conditions, the oxide protects the aluminum from further corrosion.

An exception is found in seashore locations.



When exposed to damp, salty air, most aluminum alloys behave very actively.

Sea salt (mostly sodium chloride) destabilizes the normally protective oxide film,

leading the localized attack, or "pitting." The reaction is so strong that a thin-

gauge aluminum sheet will show perforation after being immersed in warm salty

water for only a short period of exposure. However, not all aluminum alloys react

so strongly to salt air. Aluminum masts, for example, are very popular on

sailboats, but the alloy found in most aluminum flashing, roofing and siding does

not stand up to salt, and should not be used near the sea. Aluminum performs

much better in industrial atmospheres, although the top choices there are lead

and copper.



Lead also forms a surface film of corrosion when exposed to the air. Because

this film bonds so tightly with the underlying metal, however, it becomes a barrier

to further corrosion. The types of films that form on lead include sulfate, oxides,

and carbonates. Lead reacts with sulfur-bearing industrial atmospheres to

produce lead sulfate, so it becomes very corrosion-resistant in industrial

atmospheres and in areas subject to acid rain.



The green patina seen on older copper structures is a corrosion product

consisting of copper sulfates and copper carbonates. The presence of sulfate

films means that copper, like lead, holds up well in industrial atmospheres. But

there is evidence that atmospheric corrosion of copper, while low, is increasing.

Some old-timers remember that the green patina used to take about 25 years to

form. It now forms in about 10 years, showing an increased corrosion rate in the

underlying metal (although the green patina protects the underlying metal, it does

not completely stop the corrosion). Observations of Christ Church in

Philadelphia, for example show that its more than 200-year-old copper roof has

an annual corrosion rate lower than that seen in contemporary structures.



While lead and copper serve well in heavily industrial atmospheres, zinc and

galvanized steel fare poorly under the same conditions. Unlike aluminum,

however, zinc and galvanized steel are the metals of choice in seacoast

locations, where they suffer little damage from salt-heavy air.



Uncoated iron and steel are quite a different story. Although they are ranked

midway in the EMF Series, indicating that they’re mildly active metals, they are

next to aluminum in the Galvanic Series for seawater. The active behavior of iron

and steel results from the type of native oxide that they form. In contrast to the

dense, tightly adhering films associated with aluminum, lead and copper, iron

and steel oxides tend to be loose, porous, and nonadhering. The oxide flakes off

almost as soon as it forms, exposing a fresh metal surface to further oxidation

and attendant loss of metal.



An exception to this are products developed called "weathering steel" which

modifies the oxide by alloying steel with copper to make the surface film more

adherent, thus providing protection. Weathering steel will corrode, but it will

usually do so more evenly and at a much slower pace than steel.



It cannot be overemphasized that the corrosion resistance of a metal depends on

its naturally forming surface film, as well as on whether or not the film is

protective. But corrosion is a complex subject, and several variables can

influence a particular metal’s performance. Local experience in different regions

with each material is usually the best guide to its suitability for a particular use.



Galvanic corrosion - The Galvanic Series assume freely corroding metal,

unaffected by contact with any other substance. Galvanic corrosion is a form of

electrochemical corrosion that occurs when two dissimilar metals come together

in the presence of an electrolyte to form an electrical couple, known as a galvanic

couple. In building systems, the electrolyte is usually ordinary moisture, whether

rainwater of high atmospheric humidity.



When two metals form an electrical couple, an exchange of electrons takes

place, its direction and intensity governed by each metal’s ranking in the

Galvanic Series. The farther apart the two metals are on the Galvanic Series, the

greater the potential for corrosion (see Table 2). This exchange protects the

more noble (less active) metal, while causing the more active metal to corrode

even faster. The more active metal gives up electrons, sacrificing itself to protect

the more noble. We call the active, corroding metal the "anode" and the noble,

non-corroding metal the "cathode". After the anode corrodes completely away,

the cathode will again begin to corrode as reflected by its position in the Galvanic

Series.

Although builders rightly see galvanic couples as something to be avoided, the

process has its uses. Boaters, for example, use sacrificial anodes - buttons or

bars of an aluminum magnesium alloy that corrode instead of more desirable

metallic boat parts - to protect engine parts or propellers. And galvanic couples

are the mechanism by which galvanizing works.



Galvanizing means simply overlaying steel with zinc, either by plating or by

dipping the steel in molten zinc. An undamaged piece of galvanized steel will

corrode at the same rate as a similar piece of zinc. Once the zinc coating is

perforated (by mechanical damage, for example), the zinc forms a galvanic

couple with the steel, the zinc corroding to protect the steel. The zinc will

continue to protect the steel until most of the zinc is gone.



When the zinc is gone, you may begin to see a lot of thin patches of rust. What

this means depends on whether the zinc was applied by plating or dipping. On an

electroplated surface, such as a galvanized-metal roof, the rust indicates that

corrosion of the underlying metal has begun. On a hot-dipped galvanized

surface, however, the zinc actually diffuses partway into the steel. The initial

patches of rust mean that the pure zinc overlay has corroded away. Thus a piece

of hot-dipped galvanized steel will give you some warning before the steel begins

to corrode.



By Ana Diaz



Condensed from Fine Homebuilding August/Sept. 1990



NOTE: See CLI technical capabilities in corrosion evaluation, consulting and

failure analysis.

                                       TABLE 1



                         The Galvanic Series for Seawater



                           Graphite



Most cathodic, or          Gold

passive

                           Silver



Most anodic, or active     Passive stainless steel



                           Nickel



                           Siler Solder



                           Copper



                           Brasses



                           Tin



                           Lead



                           Lead-tin solder



                           Active stainless steel (most stainless steel fittings)



                           Cast Iron



                           Wrought Iron



                           Copper Steel



                           Carbon Steel



                           Aluminum



                           Magnesium and magnesium alloys Zinc and galvanized iron

                           and steel

                                                      TABLE 2

Galvanic corrosion potential between common construction metals



                     Alum.      Brass       Bronze      Copper      Galvan       Iron/   Lead   Stain   Zinc

                                                                    Steel        steel          steel



Aluminum                        1           1           1           3            2       2      3       3



Copper               1          2           2                       2            1       2      1       1



Galvanized steel     3          2           2           2                        2       3              3



Lead                 2          2           2           2           3            3              2       3



Stainless steel*     3          1           1           1           2            2       2              1



Zinc                 3          1           1           1           3            1       3      1



    1. Galvanic action will occur with direct contact.

    2. Galvanic action may occur.

    3. Galvanic action is insignificant between these metals. * Active stainless steel