Prevention of Galvanic Corrosion in the Roofing Industry
Prevention of galvanic corrosion requires an understanding of material compatibility. There are three conditions that must exist for galvanic corrosion to occur.
- There must be two electrochemically dissimilar metals present.
- There must be an electrically conductive path between the two metals.
- There must be a conductive path for the metal ions to move from the more anodic metal to the more cathodic metal.
If any of these three conditions do not exist, galvanic action will not occur. Galvanic action can be minimized by design. Corrosion engineers have found the following practical rules.
- Select combinations of metal which be in electrical contact as close together as possible in the galvanic series.
- Electrically insulate from each other metals from different groups whenever practical. If complete insulation is not possible, paint or plastic coating at joints will help.
- If you must use dissimilar materials well apart in the series, avoid joining them by threaded connections as the threads will probably deteriorate excessively. Brazed or thermal joints are preferred, using a brazing alloy more noble than at least one of the metals to be joined.
- Avoid making combinations where the area of the less noble, anodic metal is relatively small compared with the area of the more noble metal.
- Apply coatings with judgment. Example: Do not paint the less noble metal without also painting the more noble; otherwise, greatly accelerated attack may be concentrated at imperfections in coatings on the less noble metal. Keep such coatings in good condition.
- Consider use of cathodic protection
Galvanic Corrosion is an electrochemical process which occurs when dissimilar metals are in contact with each other in the presence of an electrolyte, most commonly being moisture and oxygen. For example rain water and salt water make especially good electrolytes. An electrolyte could be any non-metal matter that will conduct an electric current and are predominantly liquids or moisture. Every metal has been rated for nobility and then placed on galvanic scales according to nobility. Basically nobility is a measurement of the resistance to corrosion, especially of one metal contacting another metal. Metals that are the least noble means they are very anodic, electropositive or high potential and will corrode most easily. Whereas metals that are the most noble, means they are highly cathodic, electronegative or low potential and will be the most resistant to corrosion. The most corrosive effects will occur between metals from the opposite ends of the galvanic scale or ranking of nobility. This difference in electrical potential between two or more metals causes galvanic corrosion. Dissimilar metals in contact with each other in the presence of an electrolyte causes current to flow through their points of contact at the expense of the metal with the higher potential or less nobility. The much less noble metal is gradually consumed in the Electro-chemical reaction and will deteriorate or wear away as the metal ions migrate away from the very anodic metal to the more noble cathodic one. The more noble metal’s corrosion resistance actually increases from this transfer of ions to it from the less noble metal, while the other metal is gradually getting consumed. Below is an abbreviated galvanic scale showing ranking of the most common construction metals from the least noble up to the most noble in a normal construction setting where both moisture and oxygen are present.
Here is a simplified chart on the affects of dissimilar metals:
0 – No reaction • 1 – Galvanic Action Will Occur |•2 – Galvanic Action May Occur • 3 – Galvanic Action is insignificant
Galvanic Scale (Nobility of Common Metals)*? This scale shows metal that are most protective (Least Noble) to Least Protective (Most Noble)
There are other terms that are used that indicate least noble (Active) and most noble (Passive)
Note: Did you know that if you take a 1” square of copper and a 1” square of galvanized metal and place a cloth dipped in salt water in between them, it will produce ¼ volt of electricity.
Electropotential of Various Metals; Listed High to Low
Since dissimilar metals in contact with each other in the presence of oxygen and moisture will result in the less noble metal being corroded or eaten away the more noble one, contact between dissimilar metals should be avoided. For example, copper as one of the most noble of metals should be kept separated from less noble metal such as aluminum and galvanized steel. If contact between dissimilar metals cannot be avoided, their surfaces should be insulated as much as possible at the least with a non-metallic, non-conductive coating, such as bituminous or zinc chromate primers or paint. Separating the dissimilar metals with tape, gaskets, waterproof membrane, sealants or other non-conductive material that does not absorb and hold moisture can be used effectively. Care must also be taken to avoid the liquid wash run off or discharge from the drains from noble metals to less noble ones. For example rain water run off from a copper roof going into aluminum gutters and downspouts will result in traces of copper salts in the run off wash accelerating corrosion of the aluminum. Additionally, the use of galvanized nails, screws, flashing or drip edge on copper roofing or copper gutters should likewise be avoided as such less noble fasteners and metals will corrode rapidly. While rain water is a good electrolyte so too is high humidity or moisture laden air especially when it contains a high concentration of corrosive acids as is typically found in larger cities or near industry. In industrial or heavily populated areas, harmful electrolytes are formed by the absorption of gases by rain and fog to form acids and salts, while in coastal areas a saline electrolyte is formed by the combination of salt with the moisture laden air. ? ?As copper has one of the highest galvanic numbers or nobility of construction metals, copper will not be harmed by contact with any of the common metals. However, copper will promote corrosion of other less noble metals if placed in direct contact with them. Copper can be used most appropriately with lead, tin, lead-tin solder or stainless steel under almost any circumstance. The primary metals of concern in terms of galvanic corrosion contact with copper are aluminum and zinc. Large differences in the thermal expansion rate between copper and these metals may cause additional problems. Also avoid using copper and some brass fasteners with aluminum, zinc and most ferrous metal. Paints or coatings that are used for isolation must be compatible with both of the dissimilar metals. The bituminous or zinc chromate primers and paints can be used between copper and aluminum surfaces. These same coatings or a red lead primer can also be effective in separating copper from iron and the other ferrous metals such as galvanized steel or mild steels.??Another type of corrosion called erosion corrosion is caused by the flow of acidic water concentrated into a very small area. While especially true for the least noble and most corrosive of metals, but capable of affecting all metals, acidic water allowed to concentrate on a small area, for example rain water from a large area like a roof diverted towards a flashing, valley or gutter can cause erosion corrosion. Also acidic water or liquids pooling on metal surfaces or trapped without air circulation between roofing or sheathing can quickly corrode the metal. Solution is to eliminate such pooling, maintain air circulation underneath metal roofs or between inert roofing material and drip edge, valleys and flashing. Another solution is to use replaceable reinforcing insulative strips between the inert materials and metal. Raw zinc and aluminum especially are susceptible to prolonged contact with acidic moisture, as for example the underside of a zinc roof. But care should be taken even with copper and stainless steel. Copper and zinc possess a superior property in that both will gradually develop a long term protective patina that helps guard against such corrosive effects for a very long period of time providing initial safeguards are exercised such as drying out provisions or temporary coatings.
*Note: Galvanic chart rankings involving sea water immersion and low oxygen conditions will vary somewhat – the more typical nobility given here is for land based construction.
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