Utilizing Corrosion Coupons to Optimize Steel Corrosion Control Program
South Central Illinois Groundwater System
This case history is an excellent example of how corrosion coupon monitoring can help minimize steel corrosion rates and reduce red water complaints. It also points out the value of working with a knowledgeable consultant that provides service on a regular basis.
Background
The municipality utilizes a lime softening process that provides a finished water that is relatively corrosive. The final hardness is 60-85 mg/L and total alkalinity is approximately 100 mg/L, both as CaCO3. The process removes the iron and manganese coming from the aquifer, but causes corrosion in the cast iron pipes in the distribution system.
The municipality’s previous consultant recommended their product to be fed at approximately 1.0 mg/L as PO4. They fed the product for two years with minimal success. The city on their own cut back the feed rate to 0.5 mg/L and felt that the complaints were reduced. At this point the superintendent didn’t know exactly what to do next because they weren’t getting any assistance from the consultant.
Recommendations
WSU recommended using corrosion coupons in addition to the number of customer complaints to gauge their progress. Complaints by themselves were too subjective. A coupon rack was placed at the treatment plant and at a booster station out in the system.
WSU also recommended a blended phosphate inhibitor (WSU 318) that would their research indicated should be fed at a higher feed rate. The coupon data would help them determine the optimum feed rate. Regular service visits were also recommended.
Corrosion Monitoring
The system was first monitored with coupons while the city was still feeding the old phosphate product at the 0.5 mg/L feed rate. The corrosion rate was 10.52 mils per year (mpy). The chemical was then turned off and the system was monitored for 90 days. The corrosion rate was 14.83 mpy. The customer complaints were reduced when the chemical was shut off, but the corrosion rates increased. WSU explained why this was happening and why the higher feed rate would correct this.
WSU Program
WSU began by feeding their product at 1,25 mg/L. Corrosion rates went down to 8.25 mpy. It took four months before complaints were experienced. Feed rates were increased to 2.0 mg/L (3.5 mpy). The city could then go between the spring and fall flushes with negligible complaints. Current feed rates are 2.5 mg/L as PO4.
Now, even the flushes take one-half the time, saving labor and money. This also indicates that less iron oxide is being produced, which correlates to the lower corrosion rate. There are even fewer complaints during the system wide flush itself. WSU provides service every 5-6 weeks to answer questions and monitor overall progress.
Summary
This case points out several important distinctions about Water Solutions Unllimited:
We have an excellent understanding of:
1. Steel corrosion inhibition and how to determine correct feed rates.
2. How to use corrosion coupons to monitor corrosion rates and system progress.
3. The need for continuing service to answer questions and monitor performance.