Iron containing a high silicon percentage was developed in the early 1900’s. The cast material was extremely hard and brittle. It was first seriously considered for impressed current anode application in the early 1950’s. It was introduced as an anode material in 1954. A subsequent modification to the alloy in 1959 produced better anode performance characteristics. This alloy consisted of the addition of 4.5% chromium. This anode material has been widely used and accepted in the industry.
High silicon chromium cast iron is a solid, non-porous material. This alloy consists of a matrix of silico-ferrite in which the majority of the carbon is in the form of graphite flakes at grain boundaries. Adding chromium results in eliminating graphite.
The produced cast iron material used for anodes typically has the following mechanical properties:
Maximum resistivity 72 micro ohm-cm
|Mechanical Strength:||Compression – 100000 pounds per square inch|
Flexural – 15000 pounds per square inch.
|Coefficient of thermal expansion:||0.72 x 10-6/F|
The standard metallurgical composition of cast iron anodes conforms to ASTM Standard A518-86 Grade 3 as follows:
This alloy is cast by several methods including sand mold casting, chill-casting, and centrifugal casting. A variety of anode shapes and sizes are available. The most common anode shapes are cylindrical tubes and solid bars in lengths up to 84″, diameters from 1″ to 6″, and weights up to 280 pounds. The standard length for the solid bar anodes is 60″. The standard length for tubular shapes is 84″.
Each cast iron anode is normally provided with an individual cable of varying length. Cast iron anodes are provided in both end-connected and center-connected configurations. The solid bar anodes are cast with a hole at one end to accommodate a connecting cable. Center-connections are used for cylindrical tube shapes. There have been numerous methods and procedures for connecting cable to cast iron anodes. The most common connector for solid anodes is a poured and tamped lead connection in the cast hole. Center-connected anodes utilize a one or two piece lead assembly attached to the interior center of the anode.
Following cable connection, the annular space around the cable is filled with a high quality electrical sealant. Common sealants are asphaltic electrical potting compounds. Care must be exercised to insure the compound is at the proper pouring temperature and that there are no voids or air pockets within the cavity. Anode caps such as epoxy or heat shrinkable caps are commonly used for additional protection. Cast iron anodes can be prepackaged in steel canisters with carbonaceous backfill. Common canister sizes are 8″ x 72″, 8″ x 84″, 8″ x 96″, 10″ x 84″, 10″ x 96″, 12″ x 84″, and 12″ x 96″.
The reported consumption rate is between 0.2 and 1.2 pounds per ampere-year. The controlling factor appears to be the environment. Manufacturer recommendations for anodes surrounded by carbonaceous backfill is 0.7 pounds per amp-year. Current densities should be limited to approximately 1 ampere per square foot.
High silicon cast iron anodes are widely used in underground applications in both shallow and deep groundbeds. Although the performance is improved with coke breeze; its use is not critical. This material is also widely used in freshwater and saltwater environments.
The performance of cast iron as an anode is dependent upon the formation of a thin layer of silicon dioxide on the surface of the anode. Oxidation of the alloy is necessary to form this protective film. Silicon-chromium cast iron is highly resistant to acid solutions. It does not perform particularly well in alkaline environments or in the presence of sulfate ions.
There have been some reports of early failure when silicon iron anodes are exposed to environments in which both sulfate and chloride ions are present. Other cases are reported where these anodes increase significantly in resistance when exposed to drying conditions. It is thought that this condition interferes with the formation of the conductive silicon dioxide film.