|Content Type||Conference Paper|
|Title||Current Demand for Cathodic Protection of Coated Steel - 5 Years Data|
|Authors||Ole Oystein Knudsen and Unni Steinsmo, SINTEF Materials Technology|
|Source||CORROSION 2001, March 11 - 16, 2001 , Houston, Tx|
|Copyright||2001. NACE International|
|Keywords||Cathodic protection, organic coating, coating quality, current demand, cathodic disbonding, coating breakdown|
The current demand for cathodic protection of coated steel samples have been measured for 5 years. Organic coatings of various thickness and type have been tested, representing different classes of coatings according to DNV RP 401 and NORSOK M501. The samples were exposed in natural seawater and polarized with zinc anodes. For samples with only a primer coating (50 lure thick) the current demand started to increase immediately after immersion. However, after a few months the current demand stabilized and it has been stabile since. The current demand after five years was 10 - 20 % of the value for bare steel. For samples with 150 lam thick coatings the current demand increased during the first two - three years of exposure, but was stabile since. The current demand for these coatings after five years was 3 - 6 % of the value for bare steel. For samples with 450 ~tm thick coatings or more the current demand has been low during the five years of exposure. For one of the coatings the current demand started to increase during the last year, but the current demand was still below 1% of that for bare steel. Cathodic disbonding around mechanical damage in the coatings does not seem to affect the current demand. Blistering due to cathodic reduction of oxygen under the coating seems to be the most important factor for the current demand. A conceptual model for predicting the current demand as function of coating thickness and time is suggested.
Several oil companies have reported successful use of organic coatings in combination with cathodic protection of submerged structures. 1-6 There are several advantages with this approach. The coating ensures an even current distribution and rapid polarization of the structure, but most importantly the total anode weight can be reduced significantly. According to Det Norske Veritas (DNV RP B401)7 the anode weight can be reduced by 50 - 70% depending on the coating and the design life, compared to an uncoated structure. According to the NORSOK M503 8 standard a 60 - 95% reduction in anode weight is allowed.
The major uncertainty with using coatings in combination with cathodic protection is the degradation rate of the coating. Degradation of the coating, such as physical damage in the coating, blistering and cathodic disbonding may increase the current demand for cathodic protection. This must be taken into consideration during design of cathodic protection. Several models for predicting the increase in current demand with time are available. 9 However, few long term data to verify these models have been presented. In addition, most long-term experiences are with epoxy-tar coatings, which now have been replaced by new and possibly better products. The recommended degradation rates for the coatings may therefore be quite conservative, resulting in unnecessarily high anode weights.
In a previous paper, current demand for cathodic protection of coated samples after two years exposure was presented, l0 This paper presents the current demand after five years exposure. The correlation between the ASTM-G811 test for cathodic disbonding and cathodic disbonding after five years exposure in natural seawater is also presented.
The steel specimens were prepared from 3 mm (0.1 in.) hot-rolled steel (DIN 17100). Prior to application of the coating the samples were blast cleaned with aluminum silicate to Sa 2 1/2 and medium roughness (Ry = 48 lam, 1.9 mils). The coatings were applied with an airless high-pressure spray gun in one or two coats to the desired film thickness. All coatings cured for 26 - 28 days before exposure. Description of the coating systems and dry film thickness of the samples are
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