Materials Performance is the world's most widely circulated magazine dedicated to corrosion prevention and control. MP provides information about the latest corrosion control technologies and practical applications for every industry and environment.
Issue link: http://mp.epubxp.com/i/852556
19 NACE INTERNATIONAL: VOL. 56, NO. 8 MATERIALS PERFORMANCE AUGUST 2017 Information on corrosion control and prevention sea level. 5 After the autoclave exposure, the coated surface of the specimen was examined by light optical microscopy, scanning electron microscopy (SEM), and energ y dispersive x-ray spectroscopy (EDX). The corrosion rate was calculated by determining the amount of dissolved aluminum in the seawater after testing. Since aluminum and iron salts were not originally present in the synthetic sea- water, all A l 3+ and Fe 3+ ions in the solution after testing were presumed to originate from the sample. Photographs of the TSA-coated speci- men before and after exposure to the syn- thetic seawater indicate the exposed steel in the defect (holiday) region did not exhibit signs of any iron-based corrosion products from the steel; however, white corrosion product—aluminum hydroxide [A l(OH) 3 ] from the TSA—was evident on the surface of the intact TSA coating. SEM of the holiday cross section showed a calcareous scale deposit on the exposed steel surface that varied from a few hun- dred µm near the TSA interface to a few tens of µm near the center of the holiday region, and also conf irmed there were no visible steel corrosion products. A detailed examination using EDX showed the protective multilayered deposit was comprised of magnesium and calcium compounds. According to Paul, the aluminum in the TSA coating acts as the anode and polarizes the exposed steel surface in the holiday area. The polarization of exposed steel leads to the production of hydroxyl ions (OH – ), which increase the pH at the steel/seawater interface. In turn, the OH – ions and high pH (>9) facilitate the pre- cipitation of magnesium hydroxide [Mg(OH) 2 ] from the seawater. "Seawater has a lot of magnesium and calcium ions, and the precipitation of one or the other depends on their relative solubility," Paul explains. "If the solubility of the com- pounds in the seawater is sensitive to local pH, then they will precipitate out at certain pH levels and form the deposits." He notes that the composition of the calcareous deposits formed on a cathodi- cally polarized steel surface also depends on temperature. The Mg(OH) 2 is depos- ited at low temperatures and ambient pressures (≤5 °C) while the calcium car- bonate (CaCO 3 ) deposits are formed at ambient temperatures (20 to 25 °C) and ambient pressures. With increasing tem- peratures, he says, solubility of CaCO 3 in water decreases while solubility of Mg(OH) 2 increases. During the 90-day test, as the Mg(OH) 2 precipitated at the steel/sea- water interface, the pH away from the interface decreased to a value where Mg(OH) 2 could no longer precipitate. Therefore, the calcareous deposit next to steel consisted of a ~200-µm thick layer of predominantly Mg(OH) 2 , while the deposit formed away from the steel was mainly CaCO 3 . This deposit behavior is very similar to calcareous deposits observed for ambient temperatures and pressures. The formation of calcareous deposits on exposed steel surfaces during CP of subsea structures is generally regarded as favorable since it reduces the cathode area and consequently reduces the corrosion of exposed steel, Paul says. There was no evidence of CS corrosion products in the calcareous deposits, Paul adds, which led to the conclusion that it is very likely the damaged TSA will protect a CS surface from corrosion under the pressure conditions found in deep seawater. Using data from the 90-day test, the calculated corrosion rates were 0.0002 mm y –1 for the aluminum in the coating and 0.002 mm y –1 for the CS substrate. Paul comments that these values are low and the uncertainty and complexity asso- ciated with extrapolating corrosion rates from dissolved ion concentration data alone over a short test period dictates that the values should be treated with caution. Nonetheless, he says, formation of calcareous matter in the holiday region and the high integrity of the TSA coating indicates that it can potentially provide long-term corrosion protection to off- shore CS structures in deep sea environ- ments, even when the TSA coating is damaged in certain regions. Contact Shiladitya Paul, TWI—email: Shiladitya.Paul@twi.co.uk. References 1 " What is T.S.A?" Metalink Metallizing Sys- tems, Apr. 30, 2010, http://www.metalink- wear.com/assets/knowledgeBase/What%20 is%20TSA.pdf ( July 6, 2017). 2 N. Ce, S. Paul, "Thermally Sprayed Aluminum Coatings for the Protection of Subsea Risers and Pipelines Carrying Hot Fluids," Coatings 6, 58 (2016). 3 " How a re su b s e a ga s pi p e li n e s b u i lt ? " G a s p r o m In f o r m a t o r i u m , h tt p : / / w w w. gazprominfo.com/articles/undersea-tubes/ ( July 6, 2017). 4 ASTM D1141-98, "Standard Practice for the Preparation of Substitute Ocean Water" (West Conshohocken, PA: ASTM Interna- tional, 2013). 5 "Water Pressures at Ocean Depths," National Oceanic and Atmospheric Administration, https://www.pmel.noaa.gov/eoi/nemo1998/ education/pressure.html ( July 6, 2017). —K.R. Larsen MP — Coupons for Cathodic Protection Evaluation of Mixed Metal Piping Systems Hydrogen Sulfide Tube Failure in Seawater Heat Exchanger Electrochemical Approach for Screening Unknown Corrosion Inhibitors in Simulated Pore Solution Rust Layer Characterization of Buried Drinking Water Pipe Plasma Electrolytic Oxidation Coatings for Aluminum Alloys