Journal Archive

Platinum Metals Rev., 1980, 24, (2), 56

New Anode for Corrosion Protection

Platinum Niobium to be Utilised in Arid Environments


  • By P. M. Sly
  • Metal and Pipeline Endurance Limited, Stotfold, Hertfordshire

The protection of steel structures from environmental corrosion by means of cathodic protection systems is not new, and the use of platinum for this purpose has been reported in this journal on several occasions (1). However many factors have to be considered when designing such a system if it is to be technically and economically successful for the particular application—and in the local environment—it is intended for. This article reports the development of a new system for the cathodic protection of oil well casings and pipelines in arid environments.

As the upper strata of the earth in dry areas of the world has a high resistivity it is not practical to use conventional surface anode beds there, and for many years this problem has been overcome by installing anodes at considerable depth, within lower resistivity strata. Until the development reported here the usual practice was to drill, and line, a hole 0.3 metres in diameter down to the static water level, which in the Middle East is found at an average minimum depth of 150 metres. Drilling would then be continued to accommodate the requisite number of anodes in order to achieve the desired current output within this lower resistivity strata. Installation costs were formidable and maintenance was difficult.

In order to produce an output of 100A, the anode column would consist of 20 conventional anodes, separated vertically by a distance of 0.5 metres. Thus the hole drilled had to penetrate typically 40 metres below the static water table, incurring substantial drilling charges. In addition, because the water downhole is not an ideal environment for these anodes, a special low resistance coke breeze slurry had to be pumped down the hole, to provide a lower resistivity environment for optimum anode current output and to consolidate the anode column.

Typical 100 Ampere Deepwell Installation in the Middle East A Comparison of Platinised Niobium vs Conventional Anode Materials

Conventional Groundbed Materials Platinum Clad Niobium
Borehole diameter 0.3 metres 0.15 metres
Borehole length below static water level Greater than 30 metres Maximum 6 metres
Coke breeze backfill Necessary Not required
Electrical cable Individual cables to each anode One cable
Maximum operating voltage 20 volts 100 volts
Maintenance Very difficult Easy
Life 10 to 15 years Greater than 25 years
Present overall cost of installation 150,000 U.S. Dollars 50,000 U.S. Dollars

Now Metal and Pipeline Endurance Limited have completed on behalf of aramco, the Saudi Arabian Oil Company, the development of a new cathodic protection application for platinum clad niobium anodes in deep groundbeds. The anode consists of a solid niobium rod 12 mm diameter and clad with 0.01 mm of platinum. Again, the length of the anode depends on the required current output and they have been constructed in lengths of up to 6 metres, by resistance butt welding 1.5 metre lengths. The anode, which is enclosed within a PTFE cage to provide protection during installation, is suspended from the surface of the deepwell, and electrical contact is achieved by a chlorine resistant anode cable which is clamped to the suspension rope at intervals up to ground level.

These platinum clad niobium anodes only require a drilled hole 0.15 metres in diameter, and for an equivalent capacity of 100A an anode length of 6 metres is satisfactory. As a result, the overall drilling costs are of the order of one third of those for drilling conventional deepwell groundbeds. In addition the basic cost of the platinum clad niobium assembly is approximately one fifth that of current deepwell anode assemblies.

Apart from basic cost savings, several other advantages result from the use of platinum clad niobium anodes in deep groundbeds. These are summarised in the Table but it is worth noting that a platinum clad niobium assembly requires only minimal equipment for its installation, removal and maintenance, and does not require the purchase or installation of expensive, special grade coke backfill.

A minimum working life of twenty-five years is expected for these new platinum clad niobium anodes which, it is anticipated, will ultimately replace all conventional deepwell anode materials.

Reference

  1. 1
    Most recently, L. L. Shreir, Platinum Metals Rev., 1965, 12, ( 2 ), 42 – 45 ; 1977, 21, (4), 110 – 121 ; 1978, 22, (1), 14 – 20

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