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Corrosion control in cooling systems

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article image ONDEO Nalco Victorian account manager Stephen Powell monitoring the water treatment program on site at 120 Collins Street, Melbourne.

ONDEO Nalco , a leader in water treatment and process chemicals, claims to have established a new benchmark for cooling system corrosion inhibition in the post-chromate era.

In 1999, ONDEO Nalco was appointed to manage a high profile 52 storey office building's open recirculating cooling water systems, as well as its hot water heating and its chilled water closed systems.

The 15,000 litre air conditioning condenser water circuit is located on the 52nd level and consists of two hydraulically linked cooling towers.

Until July 1997 chromate-based inhibitors were used with excellent results against the key performance indicator but were replaced due to environmental and OH&S concerns (chromate is carcinogenic).

The key performance indicator (KPI) is the pitting rate, which must not exceed 20mpy (ml per year), as measured on mild steel coupons and verified and cross-checked by corrator technology.

Chromate based corrosion inhibitors showed pitting rates of 8-10 mpy until replaced and had been recognised as the industry standard.

Zinc phosphate based inhibitors were introduced in the second half of 1997 and within four months the pitting rate had reached 20mpy. Within another four months this had reached 250mpy.

ONDEO Nalco was approached to conduct an extensive analytical testing program to determine the cause. Results suggested that soluble copper in the make-up water was responsible.

Protection of copper and copper alloys in condenser water circuits is well documented. Consequently water has previously been treated by the inclusion of azoles in inhibitor formulations.

However, the influence of make-up waters with a high soluble copper presence introduced a new challenge.

The exposure to copper risers delivering water from street level to level 52 provided incoming soluble copper levels of 180 parts per billion in the cooling towers make-up.

This was then concentrated to levels typically 5-8 times higher. This copper then electroplated as a visible copper film formation on mild steel components. This caused the resultant galvanic corrosion attack.

Management of the soluble copper was required and initially TT-tolytriazole in combination with a zinc phosphate based inhibitor was considered suitable to generate an extremely effective mechanism to control the excess soluble copper throughout the system.

The initial ONDEO Nalco chemical treatment program consisted of a multi-functional zinc/phosphate corrosion inhibitor with a sodium hypochlorite oxidising biocide supplemented by a non-oxidising biocide.

Manual additions of TT and zinc phosphate were made. The pitting rate immediately fell and for a short period met requirements. However it then fluctuated at two to four times allowable levels.

The cause was the strong alkalinity of TT and the strong acidity of zinc phosphate. The manual additions had created a temporary loss of pH control that had a deleterious effect on the entire water chemistry control.

To remedy the situation, peristatic pumps and timers were installed and the required quantities of TT and zinc phosphate were injected over set time intervals.

Corrosion monitoring over four consecutive quarters now showed mild steel pitting to be lower than the previous chromate based inhibitor levels.

The manipulation of water treatment chemicals in this case has set new standards for practical corrosion control that are superior to those achieved with chromates.

This project has also established that high soluble copper bearing make-up waters can be efficiently treated and that oxidising biocides, especially chlorine, do not pose a threat to asset protection if correctly monitored and controlled.

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