For protection against microbiologically influenced corrosion, it is essential to achieve a continuous protective coating barrier that will resist highly aggressive chemicals such as sulfuric acid. Also important are adhesion, abrasion resistance, cathodic disbondment resistance, impact resistance, and low permeability.

Since 1993, a 100% solids polyurethane coating, developed by the author at Madison Chemical, has been successfully used as an internal lining system for hundreds of miles of steel, ductile iron, and concrete pipes serviced in North American wastewater industry. In addition to its excellent handling characteristics such as cold temperature cure, fast setting and unlimited film build, zero VOC's, and non-leaching, the 100% solids polyurethane coating has the performance characteristics as follows.

Performance Characteristics of The 100% Solids Polyurethane Coating

• Characteristics--->Method--->Results
• Chemical resistance to 20% H2SO4--->ASTM D714, 100,000 hours continuous immersion, coated on steel and ductile iron--->No change, No blisters
• Adhesion to steel--->ASTM D 4541--->2000 p.s.i. (glue failure)
• Adhesion to ductile iron--->ASTM D4541--->2600 p.s.i. (cohesive failure)
• Adhesion to concrete--->ASTM D4541--->Exceeds cohesive strength of concrete (500 p.s.i.)
• Abrasion resistance--->ASTM D4060, CS17, 1 kg, 1000 cycles--->78 mg loss
• Cathodic disbondment--->ASTM G95--->9.9 mm radius
• Flexibility and resistance to cracking--->ASTM D522--->180o over a 2" mandrill (12% elongation)
• Impact resistance--->ASTM D2794 at 40 mils--->110 in-lbs.
• Permeability--->ASTM D1653--->(non-breathing) 0.005 perm-inches
• Salt spray resistance--->ASTM D714, 1000 hours, coated on steel and ductile iron--->0 under film corrosion

It has been proven that a 40 mils thick film of the 100% solids polyurethane coating can provide a totally holiday free layer (under holiday testing of 100 volts per mil or higher) on steel and ductile iron substrates. The above performance testing results merit the outstanding protection of these substrates against microbiologically influenced corrosion.

There is no systematic method for evaluating the performance of a coating system for a concrete substrate under both wet and dry conditions. However, the current environment of fiscal restraint and fiscal responsibility has encouraged, if not forced, municipalities to design their wastewater structures for maximum longevity. Full testing programs have therefore been developed by some municipalities to evaluate all available coatings systems for concrete and clay brick facilities. Among these programs one that should be particularly mentioned is the study conducted by the Greater Houston Wastewater Program, City of Houston.¹

protection using the 100% solids polyurethane Coating incorporated with anti-microbial agents
A modified version of the above 100% solids polyurethane system has been made. The modification is the addition of 0.1-0.5% of an anti-microbial agent into the existing coating formulation. The anti-microbial agent used is a non-toxic, stable to 125^oc (256^oF)\, UV stable material, containing no phenols, tin, heavy metals, lead, mercury, or formaldehyde. The anti-microbial agent acts as microbial 'spears' that pierce the Thiobacillus cells, inhibiting the growth of the cells. The anti-microbial agent is also chemically reacted with the coating resins and incorproated into the polymeric. This enables its permanent presence across the whole coating film and causes no-leaching to the environment.

A testing program has been set up to evaluate how the modified 100% solids polyurethane coating and coated surface is protected with the anti-microbial agent. Three most common microorganisms found in wastewater applications were tested: Thiobacillus thiooxidans, Thiobacillus thiooparus, and Thiobacillus denitrificants.

After pH adjustment with carbon dioxide, a 0.2-milliliter aliquot of a bacterial suspension of the Thiobacillus species was aseptically pipetted evenly onto the upper surface of each disc and incubated at 25^oc for 24 hours.

The upper surface of the discs were washed with 10 ml of ATCC medium 238 for Thiobacillus thiooxidans, and 10 ml of ATCC medium 290 for Thiobacillus thiooparus and Thiobacillus denitrificants. The cell suspension was then collected into a sterile container and ten-fold serial dilutions of the bacterial suspension were made in the appropriate medium to obtain viable counts using a modified NETAC method. Four replicates were made per set.

As a comparison, control samples (the unmodified 100% solids polyurethane coating) were also used to test the effect of the coating on the three microorganisms. However, as expected, no anti-microbial activity was found.

The percentage of bacterial reduction of the anti-microbial agent from the modified 100% solids polyurethane coating is summarized as follows. It clearly suggests that the anti-microbial agent will remain active after incorporation into the 100% solids polyurethane, as 99.9% of the tested organisms were killed.

% Reduction of Thiobacillus microorganisms by the anti-microbial agent after incorporated into the 100% solids polyurethane coating²

• Microorganisms--->Viable count / ml. (initial)--->Viable count / ml. (recovered from water)--->% Reduction
• Thiobacillus thiooxidans--->1 x 10⁵--->1 x 10¹--->99.99
• Thiobacillus denitrificans--->1 x 10⁶--->1 x 10²--->99.99
• Thiobacillus thioparus--->1 x 10⁵--->1 x 10²--->99.99
  

By- Shiwei Guan, Ph.D, Director, R & D, Madison Chemical Industrial Inc.,  Canada
References
 C. Vipulanandan, et al., "Evaluating Madison Chemical Industries, Inc. Product for Coating Wastewater Concrete and Clay Brick Facilities in the City of Houston", Report No. CIGMAT/UH 96-5, December 96.
C.L. Baugh, Test Report # 970061, Custom Biologicals, Inc., August, 1998