ABSTRACT

The silicon iron anodes used in impressed current cathodic protection are manufactured using three different casting methods: The centrifugal method, the die-cast method, and the sand-cast method. The sand-cast method is restricted to the production of stick anodes; therefore, for the purposes of this research, the centrifugal-cast method and die-cast method will be analysed and assessed. These two manufacturing methods results in anodes with differing microstructures of the casting that can lead to variations in the expected life of the anode. This paper will discuss, examine, and conclude on the metallurgical differences and effectiveness between the centrifugal-cast silicon iron tubular anode and Die-cast silicon iron tubular anode based on the research and testing performed and reported by Independent Laboratory Testing. Analysis of the microstructure and formation of the graphite structure will confirm that the centrifugal casting method results in a more superior structure of the anode matrix which results in a longer working life for the anode. The testing carried out to compare the centrifugally-cast and die-cast anodes include: relative density tests, accelerated corrosion tests in in 35% Hydrochloric acid solution, 10% nitric acid solution, and a 3% sodium chloride solution, and lastly Potentiodynamic testing to American Society for Testing and Materials (ASTM) 59(1) standards.

INTRODUCTION

Originated in 1908 for producing armaments, high silicon cast iron (HSCI) anodes play an important role in the protection against the destructive effects of corrosion in today’s vital infrastructures, including oil and gas pipelines, storage tanks and other on-shore and marine installations. Popular and cost effective, the semi-consumable, silicon cast iron tubular anode is an acid resistant cast iron containing 14 to 16% silicon and less than 1% carbon that is documented to have excellent corrosion and acid-resistance properties—mainly due to the stability of the silicon-oxide layer formed around the surface of the casting.1 However, it is also known to have disadvantages, HSCI demonstrates low strength and brittleness and must be manufactured using a casting process rather than a forging, fabrication or coating process. The useful application of high silicon cast iron in a cathodic protection capacity requires the control of several metallurgical features during the manufacturing process to achieve the best microstructure.2 The alloy is also required to have adequate mechanical properties. These mechanical properties are dependent to large extent on the presence of free fine graphite in the microstructure as well as an absence of porosity, gas and shrinkage within the graphite structure. An essential aspect of the manufacture of high silicon cast iron is that it is a true cast iron and contains graphite and does not solidify as steel with carbides in the microstructure.3 Early difficulties in the production process eventually led to the creation of Duriron Patent 3129095 dated May of 1963.4 Both Levelton Engineering and the Duriron Foundry in Ohio, who were early experts in the production of silicon iron, concluded that the salient factors that determined the microstructure of the alloy, and thereafter, the mechanical and corrosion performance of the anode was dependent on the graphite structure of the alloy and the necessary absence of carbides in the metal matrix. Specifically, they concluded that the corrosion performance of silicon iron was directly linked to the shape, size and form of the graphite, the absence of any damaging secondary phases of silicides or carbides, and finally, the absence of any segregation of alloys in the structure.3 The patent also recommends a minimum mechanical strength for silicon iron anodes consisting of a transverse load minimum of around 800 pounds and a deflection of 0.025 inches should be met to avoid excessive breakage of the anodes during handling and storage.4

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