Evaluating Protective Coatings for Ballast Tanks

Transcription

1 Evaluating Protective s for Ballast Tanks The lifetime of vessels, such as tankers and bulk carriers, is determined to a large extent by the condition of their ballast tanks. In recent years, serious accidents causing total failure of the hull shell plate have been caused by corrosion damage in ballast tanks. Therefore, finding adequate methods of corrosion protection with qualifying coating systems is an important issue. In, MARINTEK began a project called Corrosion Protection of Tanks and Cargo Holds. The objective was to establish guidelines and recommendations for the handling, inspection, and maintenance of tanks and cargo holds related to corrosion and corrosion protection By Helge Vold MARINTEK Sandefjord, Norway and to test and classify corrosion protection coating systems. As part of the project, which was designed to reduce maintenance and operating costs, a manual was developed to give shipowners, inspectors, and crews information about causes of corrosion and measures to protect tanks and cargo holds. This article presents the testing methods used and the coating classifications and test results obtained. Test Methods The starting point for developing and evaluating a qualification procedure for ballast tank coatings was that suitable test methods did not exist. Therefore, it was decided to find methods that could cover all the environmental conditions in a ballast tank. To evaluate and classify ballast tank coatings, two laboratory test methods were used: a wave tank (simulating real service conditions in ballast tanks) and a condensation. PCE June s, simulating service conditions in ballast tanks, were one of two test methods used. (Photos courtesy of MARINTEK) Wave Tank The wave tank test consists of a tank with natural seawater at C and room for four test panels (Fig. ). The test panels represent one, one side panel with cold wall effect (temperature gradient about C), one side panel without cold wall effect, and one bottom panel with zinc anode. The tank has wave movement to simulate the splashing of seawater, and it can be heated by radiation on the ( hours at C/ hours at room temperature). The test cycle is two weeks with seawater and wave movement and one week with seawater below the bottom panel and no wave movement. Chamber This test consists of a in which test panels are exposed to Copyright, Technology Publishing Company

2 Table : Test Panel Description continuous condensation, according to ISO. The Test Panels The coatings were tested on prerusted, blast-cleaned, zinc shopprimed, or pre-rusted/moist test panels. For uniformity, the pre-rusted panels were produced by exposure of blast-cleaned panels to four cycles of hours in a salt spray cabinet and six hours drying with some light wire brushing before each exposure. Before application of the coatings, the pre-rusted panels were wire brushed and subjected to highpressure washing according to the coating supplier s recommendation. The blast-cleaned panels were grit blasted to Sa (ISO /). The zinc shop-primed panels were shot blasted to Sa, coated with a - µm layer of zinc ethyl silicate shop primer, weathered outdoors for days, washed with tap water, and dried. The pre-rusted/moist panels were prepared by the same pre-rusting procedure as above. One-third of each panel was coated with a coal tar. Immediately before application of the coating, the panels were dipped in distilled water and then stippled with a cotton cloth. Table shows the placement of the panels for the different tests. The test duration for both laboratory methods was days for coatings on pre-rusted panels and days for coatings on blast-cleaned or zinc shop-primed panels. Categories of s Depending on their properties, the coatings used in the tests were divided into three categories: soft coatings, semi-hard coatings, and hard coatings, which are defined as Copyright, Technology Publishing Company Laboratory Test Method Hard s B- B- B- B- B- B- TEST PANELS SEAWATER C HEATING CYCLIC HEATING HOURS C HOURS C WAVE MOVEMENT PCE June COOLING TEMP. GRADIENT Fig. : with four test panels in simulated real service conditions Description of Panels for Test Deck wave tank panel with scribe Bottom wave tank panel with anode Side wave tank panel with cooling Side wave tank panel panel panel with scribe on blast-cleaned panel Table : ification of Hard s (Degree of blisters and rust according to ISO /- ) Area Blistered No blisters Density - either on the cold wall wave tank panel or in the condensation panels Density - on the cold wall wave tank panel and in the condensation panels Density - either on the cold wall wave tank panel or in the condensation panels Density - on the cold wall wave tank panel and in the condensation panels Density - Area Rusted Ri (%) Ri (.%) Ri (.%) Ri (%) Ri (%) Ri (/%) follows. Soft coating A coating that remains soft so it wears off at low mechanical impact or when only touched by hand. These coatings give temporary protection of rusted steel surfaces, but they must be maintained or recoated every year or two. Semi-hard coating A coating that dries flexible but hard enough to touch and walk on and does not wear off or erode with ballast water movement. These coatings give temporary protection of rusted steel surfaces. Hard coating Paint that cures or dries and provides long-term protection of steel surfaces. (Paint is used as a synonym for hard

3 Table : Test Results and ification after Exposure for Hard s (Paint) Tested on Pre-rusted Substrate (Degree of blisters and rust according to ISO /- ) Type Blisters on Panel Rust On Undercutting Pull-off from Scribe Adhesion side panel with cooling (mm) (Ref) (MPa) Coal tar Solvent-free High-solids Surface-tolerant Others (non-) (S) (S) (S) (S) (S) (S) (S) (S) (S) (S) (S) (S) (S) (S) (S) (S) (S) (S) (S) (S) (S) (S) (S) (S) (S) (S) (S) (S) local (S) (S) (S) (S) (S) (S) (S) (S) (S) (S) (S) (S) (S) Ri Ri Ri Ri Ri. (.). (.). (.). (.). (.). (.). (.). (.). (.). (.). (.). (.). (.). (.). (.). (.). (.). (.). (.). (.). (.). (.). (.). (.). (). (.). (.). (.) () B B B B B B B B B B B B ) In brackets: pull-off adhesion of unexposed reference panel. First number is blister density; number in in parenthesis parentheses is is blister size. size. coating in this article). These definitions contain some evaluation of the coating lifetime, which is reflected in the classification. Similar products are ranked according to their performance. When comparing products between the groups, a low-classified hard coating may still give better protection than a high-classified semihard coating. Similarly, a low-classified semi-hard coating may still give better protection than a highclassified soft coating. Evaluation and ification of Hard s At intervals during the exposure and after the test was completed, the coatings were evaluated for degree of blistering and degree of rusting according to ISO /-. Before exposure, one small (x cm) and one large (x cm) panel were tested for pinholes with a low voltage pinhole detector PCE June at volts. The number of pinholes was recorded. After the tests, adhesion was tested using the pull-off method (ISO ). The effect of waves, heat, and temperature gradients were evaluated visually. Disbonding of the coating from the scribe was evaluated and recorded. Flexibility was tested according to ASTM D. Three pieces from the of the wave tank and an unexposed control panel Copyright, Technology Publishing Company

4 Table : Test Results and ification after Exposure for Hard s (Paint) Tested on Pre-rusted and Moist Substrate (Degree of blisters and rust according to ISO /- ) Copyright, Technology Publishing Company Blisters on Panel side panel with cooling (S) (S) (S) were cut out. The pieces were bent degrees at,, and mm diameter, and the result was reported as minimum diameter of no paint fracture. If paint fracture was seen even at mm diameter, the degree of bend at fracture was reported. Pull-off and flexibility tests were done a minimum of two weeks after exposure. For the panel with sacrificial anodes (Table ), the current demand was calculated and the effect of cathodic protection was evaluated visually. The panels were evaluated mainly on the density of blisters and the area rusted. From the evaluation, coatings were classified from to B, with being the best rating (Table ). From different producers, hard coatings were tested on prerusted substrates. The pre-treatment of the pre-rusted panels was according to the producers recommendations. As shown in Table, only four products were classified. All four were surface-tolerant coatings. Coal tar coatings, which were believed to provide the best corrosion protection for ballast tanks, showed different performances in this study from producer to producer. Only two solvent-free products were tested on a pre-rusted substrate, and they were classified B and. In the high-solids group (more than percent volume solids), only one product was classified B. Blisters on the cold wall panel in the wave tank and on the panels in the condensation test (temperature gradients) resulted in the largest deterioration of the coating systems. Salt contamination on the pre-rusted panels and poor wetting properties of the coatings were probably the main reasons for blister formation. Six hard coatings from five different producers were tested on prerusted/moist substrates. Table shows that four products were classified. These products included a solvent-free, a high-solids, a surface-tolerant, and a urethane-based coating. Deterioration of the coating systems occurred in the form of blisters on the cold wall panel in the wave tank and on the panels in the condensation test (temperature gradients). Again, salt contamination on the pre-rusted panels and poor wetting properties of the coatings were probably the main reasons for the formation of blisters. From different producers, hard coatings were tested on zinc shop-primed or blast-cleaned substrates. The highest classification,, was given to products, as shown in Table. Surface-tolerant coatings and coal tar coatings performed best in this test. Also, in the solvent-free and the high-solids groups, percent of the products were classified. Blisters on the cold wall panel in the test tank and on the panels in the condensation test caused the largest deterioration of the coating systems. Poor wetting Rust On deck panel Undercutting from Scribe (mm) ) In brackets: pull-off adhesion of reference panel not exposed. First number is blister density; number in parentheses is blister size. Pull-off Adhesion (Ref) (MPa). (.). (.). (.). (.). (.). (.) B B properties were probably the main reason for the formation of blisters. Evaluation and ification of Semi-hard s At intervals during exposure and at the end of the test, the semi-hard coatings were evaluated for degree of blistering and degree of rusting according to ISO /-. The degree of rusting in the pictures of the standard is based on hard paint coatings, which flake during rusting. Because of their softness and good adhesion, semi-hard coatings do not flake off, and this should be remembered when comparing the exposed panels with the pictures in the standard. For all coating systems, evaluation of the degree of rusting according to the standard is done without scraping off the coating. However, the coating must be scraped off in order to see underrusting on pre-rusted, blast-cleaned, or shop-primed panels. Semi-hard coatings may develop blisters that disappear later. Development of blisters was recorded during the test period and after the exposure. Because under-rusting often develops under blisters of semi-hard coatings, this must be regarded as a defect even if the blisters disappear when the exposure is PCE June

5 Table : Test Results and ification after Exposure for Hard s (Paint) Tested on Zinc Shop-primed or Blast-cleaned Panels (Degree of blisters and rust according to ISO /- ) Type Blisters on Panel Rust On Undercutting Pull-off from Scribe Adhesion side panel with cooling (mm) (Ref) (MPa) Coal tar Bleached tar Solvent-free High-solids Surface-tolerant Others (non-) (S) (S) (S) (S) (S) (S) (S) (S) (S) (S) (S) (S) (S) (S) (S) (S) (S) (S) (S). (.). (.). (.). (.). (.). (.). (.). (.). (.). (.). (.). (.). (.). (.). (.). (.). (.). (.) > (.). (.). (.). (.). (.). (.). (.). (.). (.). (.). (.). (.). (.). (.). (.). (.). (.). (.). (.). (.). (.). (.). (.). (.). (.). (.) B B B B B ) In brackets: pull-off adhesion of unexposed reference panel. First number is blister density; number in parentheses is blister size. finished. The reported size and density of the blisters is the highest level recorded during exposure. The effect of the blisters on the substrate is studied by scraping off the coating down to the steel substrate. Before exposure, one small panel (x cm) and one large panel (x cm) were tested for pinholes with a low-voltage pinhole detector at volts. The number of pinholes was recorded. After the tests, the effect of the waves, heat, and temperature gradients again were evaluated visually. Undercutting from scribe was evaluated and recorded. For the PCE June panel with anodes (Table ), the current demand was calculated and the effect of cathodic protection on the coating (disbonding/blistering) was evaluated visually. As with hard coatings, classification of semi-hard coatings was based mainly on the degree of rusting and/or the density of blisters. If Copyright, Technology Publishing Company

6 Semi-hard s C- C- C- C- C- C- Table : ification of Semi-hard s (Degree of blisters and rust according to ISO /- ) Area Rusted Ri to Ri (-.%) on all panels Ri to Ri (.-%) on deck and side wave tank panels, other panels maximum Ri (.%) Ri to Ri (-%) on deck and side wave tank panels, other panels maximum Ri (%) Ri to Ri (-%) on deck and side wave tank panels, other panels maximum Ri (%) Ri (-%) on all panels under-rusting was revealed by scraping off the coating, the classification was lowered. From the evaluation, the products were classified from C to C, with C being the best rating (Table ). Seven semi-hard coatings from five different producers were tested. As shown in Table, no semihard coatings were classified C, and only one was classified C. Four coatings were classified C. Blisters and rust formation on several panels caused deterioration of the coating systems. Salt contamination on the pre-rusted panels and poor wetting properties probably were the main reasons for blister formation. Evaluation and ification of Soft s During exposure, the coatings were evaluated visually. After the tests were completed, the coating on each panel was evaluated and rated according to the following scale: all coating remains; no coating remains. Ratings of to were used to describe intermediate degrees of remaining coating. From the evaluation of individual panels, the products were classified from A to A, with A being the best rating (Table ). For Copyright, Technology Publishing Company More than % of area rusted on all panels Area Blistered the panel with anodes (Table ), the current demand was calculated and the effect of cathodic protection was evaluated visually. Six soft coatings from five different producers were tested. No soft coatings were classified A, but three soft coatings were classified A, as shown in Table. Soft coatings with good penetrating properties and low water solubility (meaning they are difficult to wash away) performed best in this test. Seawater wave movement partly washed off some soft coatings from the splash zone and above. The heat ( C) on the reverse side of the test steel panel simulating the tank top (deck) decreased the viscosity of some of the soft coatings and thus increased their wear. PCE June Density - Density - Density - Density - Density Density A view of the test panels in the wave tank Summary and Conclusions The test results show that only four paints of tested on pre-rusted substrates received the best classification. All four were surface-tolerant epoxies. On zinc shop-primed or blast-cleaned steel, of paints tested received the best classification. After testing more than products on different substrates, the following conclusions can be made. The testing and classification system described in this article gives shipowners and shipyards a procedure to select coating systems for protection and maintenance of ballast tanks. The accelerated ballast tank test can be used by coating suppliers as a prequalification test for testing new and existing products. Results of the testing are being used by Det Norske Veritas classification society as part of its documentation for approval of coatings for corrosion protection of ballast tanks. As a result of the testing and classification system, coating manufacturers have developed new coating systems and reformulated others to improve them. specifications can be written to define performance in accordance with the criteria, guidelines, and recommendations produced by the study. References. ISO -, Paint and VarnishesDetermination of Resistance to Humidity (Continuous ).. ISO /-, Preparation of Steel Substrates Before Application

7 Table : Test Results and ification after Exposure for Semi-hard s Tested on Pre-rusted Substrate (Degree of blisters and rust according to ISO /- ) Blisters on Panels side panel with cooling (S) (S) (S) (S) (S) (S) (S) (S) (S) (S) Ri Ri Ri Ri Ri Ri Rust on Panels side panels Ri Ri Ri - Ri Ri - -% Ri Ri Ri - Ri Ri - Current Demand (ma/m ) bottom panel C C C C C C C First number is blister density; number in parentheses is blister size. Soft Coats A- A- A- A- A- A- Table : Test Results and ification after Exposure for Soft s Tested on Pre-rusted Substrate Table : ification of Soft s Evaluation on all panels tested (all coating remains) - on all panels tested on wave tank, other panels or better on wave tank, other panels or better on wave tank, other panels or better on all panels (no coating remains) Remaining on Panels side panels Current Demand (ma/m ) bottom panel. ) The coatings were evaluated according to the following scale: all coating remains, no coating remains. The rating to is used to describe the intermediate degrees of coating remaining. ) The current demand for a good protective coating is below ma/. For bare steel, the current demand is from to ma/m, depending on temperature and seawater flow velocity. of Paints and Related Products Visual Assessment of Surface CleanlinessPart : Rust Grades and Preparation Grades of Uncoated Steel Substrates and of Steel Substrates After Removal of Previous s.. ISO /-,, Paint and A A A A A A VarnishesEvaluation of Degradation of Paint s Designation of Intensity, Quantity and Size of Common Types of DefectPart : General Principles and Rating Schemes; Part : Designation of Degree of Blistering; Part : Designation of Degree of Rusting.. ISO -, Paints and VarnishesPull-off Test for Adhesion.. ASTM D-, Standard Test Method for Flexibility of Prepainted Sheet. Acknowledgements The author acknowledges the cooperation of shipowners, coating producers, and other participants in the project. The participants contributed both technical information and financial support to the project. About the Author Helge Vold is a senior research engineer at MARINTEK, Laboratory of Materials Application, Sandefjord, Norway. He has a M Sc. in chemical engineering from the Norwegian University of Technology, Trondheim, and years of experience in corrosion protection in marine environments, including field and laboratory testing of coatings. PCE June Copyright, Technology Publishing Company