pml CORROSIVITY OF SOILS CORROSIVITY OF SOILS LR-04.DOC prolonging metal life

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Donald Page
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1 i

2 ii 1.0 INTRODUCTION The causes of corrosion on underground pipework have been described in the earlier section. It was established that corrosion is a process, which is electrical in form, the rate of corrosion being proportional to the current which flows between the metal and the solution surrounding it in the corrosion cell. In underground corrosion, this solution is the soil water. There have many attempts to develop criteria for assessing soil corrosiveness. Account has been taken of acidity, resistivity, water content, analysis of the soil water, degree of aeration and presence and concentration of bacteria and coding systems have been produced. Many of these properties require laboratory, not field assessment, and the end results have no high accuracy. The simplest and most convenient check is to measure one bulk soil property - its resistivity. 2.0 SOIL RESISTIVITY The soil acts as a reservoir for water and soluble salts, the content of this reservoir being easily assessed by measuring the electrical resistance of the soil. Clay soils have a greater ability to hold water and retain soluble salts so their resistivity is low, usually less than 3000 ohm cm. Sandy soils drain more readily; the soluble salts get washed away; and so their resistivity is greater, usually in excess of 5000 ohm cm. Chalky soils often have resistivities between the two. The low resistivity clay soils are therefore the most corrosive. There are exceptions to this rule of course, for example, a sandy soil in a low lying, marshy area will be wet and show a low resistivity. It may therefore be more corrosive than many clay soils. The resistivity of the soil is easy to measure, the commonest technique in use is the Wenner Four Pin method. Four pins are pushed into the ground in a straight line (Fig. 1). The distance between neighbouring pins should always be the same. A known current is passed between the outer two pins and the resulting potential difference is measured between the middle pins. From this information, the resistance of the soil between the middle pins can be calculated. Most modern resistivity meters do this automatically and give a direct readout of the resistance in ohms. The meter reading of the soil resistance in ohms must be multiplied by a correction factor to get the soil resistivity in ohm cm. An example is shown in Fig. 1. The correction has to be made because the method measures the resistance of a different volume of soil at different pin spacings. For measurements taken from the ground surface the soil resistance (ohms) must be multiplied by 628 x pin spacing in metres, some examples are given below. Pin Spacing Multiply By Example Meter Reading x Correction = Resistivity 1.59 m ohms x 1000 = 2500 ohm cm 0.79 m ohms x 500 = 1250 ohm cm 0.40 m ohms x 250 = 625 ohm cm TABLE 1: SOIL RESISTIVITY MEASUREMENTS - WENNER FOUR PIN METHOD

3 iii There are several pitfalls that must be avoided when taking resistivity measurements. Some are shown in Fig. 2 with suggested remedies. It is best to repeat resistivity measurements if possible because ground conditions can change quickly as shown in Fig. 3. Other soil properties such as the soluble salt content and acidity can be measured on soil samples in the laboratory if needed. Soil acidity and soil aeration can also be measured on site.

4 3.0 CORROSIVITY ASSESSMENT iv Of the many systems which have been suggested for deciding how corrosive a soil really is, one of the simplest uses soil resistivity and soil type. The soil type is described in a backfill rating system: BACKFILL RATING MIXED BACKFILL RATING Sand or Chalk 1 Sand or Chalk + Cinders 3 Loan 2 Loan + Cinders 4 Clay 3 Clay + Cinders 5 Cinders 4 TABLE 2: BACKFILL RATINGS In this system any soil with a resistivity of less than 2000 ohm cm or a backfill rating greater than 3 is considered aggressive. Soils with a low backfill rating e.g. sand and a high resistivity are non-aggressive. (Note: They are not called non-corrosive because some corrosion must be expected in all soils). Soils containing industrial waster are particularly corrosive especially when they contain pieces of coal, coke, ash etc. The carbon in the coke etc., has a potential more positive than the pipe metal. A corrosion cell is therefore set up where the coke and pipe metal touch, resulting in rapid pitting corrosion on the pipe. Whenever pipes are laid in soils containing industrial waste materials a selective backfill must be used to separate the pipe from the coke etc. A pipe coating alone is not sufficient because coatings are easily damaged by rubble in made-up ground. The selective backfill is essential even if the pipework is to be protected by a cathodic protection system because very high currents will be needed to overcome the corrosive cell action between the pipe and the coke. 4.0 SELECTIVE BACKFILLS Selective backfills are needed as additional protection for the pipe and pipe coating in certain well defined situations. These are: (a) (b) (c) (d) (e) In any ground containing industrial waste materials. In rocky or very stony ground. In peaty areas, if the pipe can be laid on a firm bed. In very heavy clay soils likely to cause coating problems. (Some very heavy clay soils expand and contract on wetting and drying. This action can crack or tear coatings). In conditions where backfilling with natural soil is likely to cause coating problems (e.g. compacting a very dry clay around a pipe can damage wrapping tapes; where the natural soil cannot be compacted to give a stable reinstatement that will not subside around the pipe). Backfills should be selected to show the following properties: (a) They must be coarse enough to resist washing out (i.e. no fine building sand) but not too coarse or they will damage the coating. A size limit of 6-10mm for coal tar enamel or bitumen and 4-6mm for cold applied tape and PE sleeving is necessary.

5 v (b) (c) The backfill must be fine enough to prevent particles of ash etc. washing in and contaminating the fill. The particle size distribution of the soil must be measured in the laboratory to find the best particle size for the selective fill. A washed concreting sand is normally suitable. The backfill must be easy to use in all weathers and must flow and compact easily to give a stable reinstatement. The best backfills to use are: (1) Washed concreting sand (Zone 1 - Zone 4) (2) Fine gravel s (3mm, 6mm) (3) Coarser gravel s (10mm - 50mm) may be needed in special conditions. Sands and gravel s should normally be of quartz but crushed limestone or whinstone can be used if the particle sizes are suitable. All backfills must be well compacted especially when used underneath or beside a pipe. They should all be compacted in 150mm - 225mm (6-9 in) layers. SOIL RESISTIVITY SURVEY PROJECT No: ohm.m Severely Corrosive PROJECT: ohm.m Corrosive ohm.m Moderately Corrosiove ohm.m Slightly Corrosive Location Depth Resistance Resistivity Layer Resistiviy Corrosivity Factor R1 0hm.m Ohm.m m m ABC m m m m m m m m m m m m m m m m m

Soil Physical Properties

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