Effect of Microstructure on the Corrosion Resistance and Mechanical Properties of High Silicon Cast Iron Anodes

A paper by 

R.S. Chariton, P.Eng., BH Levelton & Associates, Richmond, B.C. Canada


As for most, if not all alloys, the corrosion resistance and mechanical properties of high silicon cast iron is affected by a number of metallurgical and microstructural factors. For high silicon cast iron these factors include:

  • Shape or form of graphite
  • Segregation
  • Presence of secondary phases such as brittle suicides and inter-dendritic carbides
  • Grain size

The following sections discuss these factors as they relate to the corrosion resistance and mechanical properties of chill cast and sand cast high silicon cast iron anodes.

Corrosion Resistance

The corrosion resistance of high silicon cast iron is attributed to the development of a thin passive barrier film of hydrated oxides of silicon on the metal surface. This film develops with time due to the dissolution of iron from the metal matrix leaving behind silicon which hydrates due to the presence of moisture. Any flaws in the barrier film will reduce its effectiveness.

The passive hydrated silicon film will bridge over and form an impervious barrier layer on a fine grained high silicon cast iron with spheroidal graphite areas much more readily than on a high silicon cast iron with coarse graphite flakes. Thus, a coarse grained high silicon cast iron that contains graphite flakes is much more likely to have structural defects/flaws in the passive film than a fine grained material with spheroidal graphite.

It is well documented that a uniform metallurgical structure normally has better corrosion resistance than a non-uniform structure. Segregation (non-uniform composition) will produce a non-uniform passive film due to varying silicon content (segregation) and the presence of second phases. In addition these can also result in localized anodic and cathodic areas on the metal surface which will result in increased localized corrosion due to the galvanic action.

Flaws in the passive film are sites for film breakdown. Penetration of the corrosive medium below the film results in localized areas of corrosion and preferential current flow due to lower resistance at graphite flakes etc. than on the hydrated silicon film.

Thus, due to the fine grain size with spheroidal graphite and more uniform composition, chill cast high silicon cast iron would be expected to have better corrosion resistance than a sand cast high silicon cast iron.

Mechanical Properties

The shape of the graphite present in an alloy affects the mechanical properties of the material. Flake graphite acts as a severe stress raiser while the spheroidal graphite does not. A classic example of this effect is the difference between gray cast iron and ductile iron.

Fine grained materials normally have higher strength and are more ductile than similar coarse grained materials. In addition, the lack of segregation and/or second phases also contributes to higher strength and ductility of materials since there are fewer areas for localized yielding and stress raisers.

Thus, a fine grained silicon cast iron with spheroidal graphite should have better mechanical properties than a high silicon cast iron with flake graphite. There are numerous references that show that this is in fact the case, although it is still a brittle material.

Relative Rate of Consumption

In order to compare the performance of "Chill Cast" High Silicon Chrome Iron anodes to the "Sand Cast" variety, Anotec ran a series of accelerated corrosion tests in accordance with Jakobs & Hewes procedure published in Materials Performance. In the first (January 1988) tests, results indicated a significant improvement in performance for Chill Cast compared to Sand Cast, especially in sulfate environments. To confirm reproducibility and to add technical and statistical credence a second series of tests were run and reported in October 1989. Results of the tests are summarized as follows:

Test: 2" diameter Specimens of each of Chill Cast and Sand Cast anodes of 0.1 1 sq m (1.2 sq ft) surface area, tested at either 2.5 Amps/sq ft or 8.3 Amps/sq ft.

Consumption g Sand Cast Chill Cast Difference
Chloride Solution 403 345 17%
Sulfate Solution 458 385 19%

Results: As shown in the table above, with a statistical confidence of 99% (ASTM G16-17) taken from Test Report October 1989.

Typical as-found anode surfaces of Chill Cast (smooth) and Sand Cast (severe pitting) are clearly contrasted in the photograph. The complete report is available from Anotec.

Relative Impact Strength

In order to compare the impact strength of "Chill Cast" High Silicon Cast Iron anodes to the "Sand Cast" variety, BH Levelton & Associates proposed a straightforward test which has been used by Anotec since 1988 to test hundreds of chill cast anodes, and some sand cast anodes from other manufacturers. The test anode is centered in a steel frame, and the end is raised as illustrated. The anode is then dropped to impact against a fixed steel anvil. Sand Cast anodes break at 2" to 4" drop. All Chill Cast anodes exceed 6"; the majority exceed 10"; and many remain unbroken at successive drops up to 13". Chill Cast strength is supported by field "survival" anecdotes.

A typical comparative test result for 2" stick anodes is shown in the table below:

Drop Height Sand Cast Chill Cast
2" (50 mm) OK OK
4" (100 mm) Failure OK
6" (150 mm) Failure OK
7" (175 mm) Failure OK
8" (200 mm) Failure OK
9" (225 mm) Failure OK
10" (250 mm) Failure Failure