Beating corrosion naturally
Nearly half a century ago it was discovered the application of zinc to ductile iron pipe helped prevent corrosion. The process observed then is the catalyst behind a more recent research and development programme by Saint-Gobain Pipelines. This research has culminated in the launch of PAM Natural, a range of ductile iron pipe and fittings for the potable water industry.
The conversion rate of zinc is directly related to the aggressivity of the soil. The more aggressive the soil, the quicker the zinc transforms. However, if the zinc is converted too quickly the resulting passive layer is of low quality. The presence of aluminium in the PAM Natural coating slows down the conversion process by forming a passive layer of aluminium oxide. This gives a more stable, protective layer and extends the life span of the zinc’s galvanic properties, even in aggressive soils.
A tough, porous, blue epoxy coating is applied at optimum thickness to permit the galvanic action of the zinc-aluminium. In addition, it makes the complete system identifiable for use with potable water.
Inka Krebel, marketing manager at Saint-Gobain Pipelines explains: “Because of the increased life span of the active protection of PAM Natural, it eliminates the need for factory-sleeved pipe, site-applied polyethylene (PE) sleeving and the application of muffs to joints in the trench. This saves time and money, as the installation process is much simpler and quicker. This one-pipe system can be used for virtually all applications and reduces the risk of choosing incorrect protection systems.”
The development of PAM Natural took nearly 15 years in both laboratory and outdoor trials. The project engineer responsible explains: “Our trials for PAM Natural included mathematical modelling and risk analysis. The research process has clearly demonstrated that PAM Natural provides a safe protection barrier and represents the latest stage in ductile iron pipe evolution.
The long response time associated with in-situ trials, even under highly corrosive conditions, led the research centre to design new corrosion tests in which the corrosive action of the soils was reproduced at an accelerated rate.
In order to simulate the normal wear and tear that takes place during handling, transportation and laying, the pipes were purposefully damaged prior to being tested.
Various juxtapositions of two different types of soils (chosen among sand, clay, limestone and peat) were set up to establish a concentration cell, either a differential aeration cell (e.g. sand-clay) or a differential pH cell (e.g. sand-limestone). The action of groundwater (permanent or fluctuating) near a pipe was also simulated.
A series of outdoor tests followed in which sections of coated pipe were placed in soils known to be highly corrosive to ferrous materials.
Clay-sand cell on bare metallized surface
This test was carried out on coupons of ductile iron metallized with either pure zinc or zinc-aluminium (ZnAl 85-15) alloy applied at a coating weight of 400g/m².
A calculation performed from the quantity of current exchanged in the cell during the first 100 days confirmed the concentration cell process transformed all available zinc. With the alloy 4 times less current was exchanged in the same time.
An examination of the same samples after 13 months revealed a significant difference in behaviour between the pure zinc and the alloy.
The pure zinc coating clearly exhibited accumulations of rust on the coupons placed in clay as well as on those placed in sand. On the alloy coating, there was no sign of red rust, merely a whitish colour indicating some transformation of the alloy. The alloy was still present in its metallic form on the coupons.
Sand-limestone cell and fluctuating groundwater on coatings
An outside laboratory performed this test to compare the behaviour of the two coatings:
- pure zinc coating, coating weight 200g/m² + bituminous sealing coat, thickness 120µm
- ZnAI 85-15 coating, coating weight 400g/m² +bituminous sealing coat, thickness 120µm.
The sand was flooded or not flooded during alternating three-week periods. Measurements of electrochemical potential and current were taken weekly.
A calculation of the quantities of current exchanged in the cells highlighted the fact that the ZnAI 85-15 alloy coating was consumed at a rate 4.3 times less current than the pure zinc coating under the same conditions.
In 1986, pipe sections treated with ZnAl 85-15 based coatings were buried at a test site at Mont Saint Michel, France, an area with especially corrosive soil, at a depth of about 1.2m. The pipe sections were placed vertically in trenches.
After a period of 14 years, the pipes exhibited a whitish shade of their original colour due to the exudation of zinc salts. Regions damaged by scraping were protected as a healing layer formed.
This test confirmed the satisfactory performance of a ZnAl 85-15 alloy-based coating in highly corrosive conditions after 14 years of exposure. Corrosion protection and damage-healing properties were visible both in the alloy and in pure zinc. Having performed well in trials in the laboratory and in situ, the new ZnAl 85-15 alloy-based coating was patented.
Selected focus groups comprising consultants, contractors and existing customers were involved in each stage of the development process and Saint-Gobain Pipelines visited every water company in the UK to gauge reaction and gather opinions which all fed into the eventual design of the product. Once the product had been tested internally, Saint-Gobain Pipelines approached the various regulatory bodies such as the WRc to prove the product could meet stringent independent tests. The new PAM Natural coating received WRc approved status in 2001.
The use of PAM Natural pipe is approved by the Secretary of State under Regulation 25 as Saint-Gobain Pipelines System XL.
PAM Natural pipe also meets British Standard EN 545 and the blue epoxy coating material is listed in the current WRAS “Water Fittings and Materials Directory” as having passed all tests in relation to its effect on water quality.
Contractor, TJ Brent and South West Water specified PAM Natural for the replacement of a 1.2km-long section of the Bears Down to St. Merryn pipeline in north Cornwall.
Project manager at TJ Brent, Ben Whayman, said: “The installation of PAM Natural was quicker and more straightforward. It was also compatible with all the traditional Tyton fittings, including anchor joints, used in steep sections of the route. Once installed, the system passed all pressure and water quality tests first time.
“We used PAM Natural to help eliminate repair costs, due to corrosion and bursting and to reduce damage caused to the environment by the repair process. We are also confident it will reduce pipe leakage and reinforce the integrity of the water supply in the area.”