Buried pipes are subject to a variety of stresses, including corrosive soil and backfill. The conventional zinc coating on the basic version of PAM pipes provides a high level of resistance to corrosion in most standard cases of application. However, corrosive soils must be assessed to determine whether additional protection, such as a polyethylene sleeve or special coatings, is required. PAM’s technical staff will carry out a soil survey at the customer’s request.
General corrosion indicators
The general corrosion indicators are determined with the aid of a detailed map (Ordnance Survey type), which indicates:
- the ground contours: high spots are drier and better aerated, therefore less corrosive; low spots are wet and unaerated, therefore likely to be more corrosive,
- water courses to be crossed, wet areas,
- ponds, marshes, lakes, peat beds and other low areas, rich in humic acids and bacteria, and often polluted,
- estuaries, polders, salt marshes and saline soils bordering the sea.
Pollution and specific corrosion indicators
Using drawings (obtained from public departments), the following are determined:
– areas polluted by various effluents, such as liquid manure, distillery, dairy, papermaking waste (etc.) or by sewage, mainly from households,
– industrial wastes like slags, clinker, etc.,
– the proximity of other mains, like leaking effluent mains,
– industrial plants or equipment using direct current electricity (cathodically protected structures, electric traction systems, plants, etc.).
This survey indicates the various strata traversed and provides information on the nature of the terrain and its natural corrosive tendency.
The following types of ground can be distinguished as a first analysis:
- low risk : sands and gravels, stony material, limestones.
- high risk: marls, clays.
- very high risk: gypsum, pyrites (iron pyrites, copper pyrites), salts used in chemical industry (sodium chloride, calcium sulphate), combustible fossil substances (lignites, peats, coal, bitumen).
Indications of the presence of fossil substances are to be noted: pyrite ammonites in particular, which indicate that the soil contains pyrites (iron sulphides) and is therefore very corrosive, particularly since it is anaerobic.
Moisture is a contributing factor in corrosive soils. A hydrogeological study identifies impermeable soils likely to retain water, as well as the presence of water retaining strata. The boundaries of these soils are often marked by the presence of springs. These boundaries warrant particular attention: the corrosiveness of the impermeable layer may be very high. The same applies for water retaining strata if they drain neighbouring soils containing soluble mineral salts (sodium chloride, calcium sulphate, etc.).
Through visual observations, measurements (resistivity) and analyses (soil samples), site surveys help to confirm and complement the topographical and geological findings.
The resistivity of a soil gives information on its ability to promote the phenomenon of electrochemical corrosion of a metal. It is a particularly significant parameter, because:
– it integrates virtually all the factors that influence corrosive factors (presence of salts, water, etc.),
– it is very easy to measure on site (Wenner, or four pins, method).
The different measurement points are taken forward on the plotting of the pipeline. The measurements are made along the provisional pipeline route, at intervals dictated by the topography of the terrain and the results obtained. The lower the resistivity, the greater the soil is corrosive. For resistivity observed below 3,000 ohm x cm, we consider it necessary to confirm the action by taking a sample at depth exposure and a measure of its resistivity (gross and minimum) in laboratory.