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Study Evaluates Offshore Coating Performance on Stainless Steel

A close-up of typical pitting seen on SS equipment in an offshore environment. Photo courtesy of Thu Tran Addis.

In the Gulf of Mexico, external pitting corrosion is a serious concern for offshore stainless steel (SS) vessels, tanks, and piping. Because of the variation in pit shapes and morphologies, it is very difficult to assess and predict the remaining service life of pitted SS pipe and equipment. An economical option to mitigate pitting corrosion on SS components is to apply a protective coating system, which can minimize pitting growth and prevent new pitting formation, as well as reduce the cost, time, and resources associated with replacing pitted SS equipment.

A study to evaluate protective offshore coating systems on pitted SS was conducted by NACE International members Thu Tran Addis, Andy Bodington, and Carmen Fonseca with BP America, Gulf of Mexico Operations (Houston, Texas, USA), and Benjamin T.A. Chang with PolyLab (Houston, Texas, USA). Their objectives were to develop a protocol for testing protective coating performance on pitted SS, evaluate how various surface preparation methods affect the coating performance on pitted SS, and determine a reliable coating application procedure to cover the pits with a holiday-free film so the integrity of offshore pitted SS is improved. Details of the study are presented in CORROSION 2018 paper no. 10487.1

For the study, two types of pits were created on Type 316 (UNS S31600) SS coupon surfaces to simulate pits seen in the field. Large pits with internal diameters of 0.062, 0.075, and 0.1 in (1.57, 1.9, and 2.54 mm) and a depth of 0.15 in (3.81 mm) were made with a drill. Small pits were created by exposing the coupons in a 6 wt% ferric chloride (FeCl3) solution in accordance with the ASTM G481.2

Type 316 SS coupons with drilled holes and pitting (left). A close-up of a coupon surface showing the pits created with a FeCl3 solution (right).

Abrasive blasting to create a 2- to 4-mil (51- to 102-µm) surface profile, which is typically done for carbon steel (CS) surface preparation, is also desirable for SS surfaces. In this case, however, the researchers were concerned that Type 316 SS vessels and piping with corrosion pits would be perforated by this level of abrasive blasting, since it can be difficult to accurately measure the depth of the pits. Instead, they used three less-aggressive surface preparation methods for the study. The first, Method 1, was a one-step method that utilized SSPC-SP 1, “Solvent Cleaning.”3 Method 2 was a two-step process that first used SSPC-SP 1 followed by cleaning with a flap disc abrasive grinder. Method 3 was a two-step process that first used SSPC-SP 1 followed by blast cleaning with aluminum oxide (Al2O3) to create a 1.5- to 2-mil (38- to 51-µm) surface profile according to NACE No. 2/SSPC-SP 10, “Near-White Metal Blast Cleaning.”4

Ten candidate coating systems—which included epoxy, epoxy/polyurethane, epoxy/siloxane, glass flake epoxy/polyurethane, phenolic novolac/polyurethane, and phenolic epoxy/polyurethane—were obtained from several coating manufacturers and applied to Type 316 SS samples, with the dry film thickness (DFT) varying with the coating system used. To determine the performance of the various coating systems, several laboratory tests were conducted.

Coating adhesion was measured before and after cyclic salt fog exposure (1 h exposure to 5% sodium chloride [NaCl] and 1 h drying at 35 °C) using a pull-off adhesion test according to ASTM D4541.5 Test results indicated that adhesion values generally were similar for the three different surface preparations when the coating systems were newly applied. However, after 8-week and 16-week salt fog test exposures, adhesion degradation was observed in two coating systems where the surface was prepared using Method 1.

Water vapor permeance is the rate of water vapor transmission through a unit area of film due to the unit vapor pressure difference between two specific surfaces. The researchers selected a water vapor permeation test to measure water permeance because all organic coating films are permeable to water and oxygen, and a coating with excellent barrier properties that reduces water vapor permeation will reduce pitting corrosion. Using a “pass” criterion of permeance <10-4 g/h/m2/Pa (according to ASTM E966), six of the 10 coating systems tested had acceptable permeance.

Because mechanical damage to protective coatings in the field is unavoidable, a coating system should maintain low underfilm corrosion or rust creepage. To measure underfilm corrosion and pit initiation, the researchers scribed test panels coated with the candidate coating systems, then tested them according to ASTM G85.7 Ten panels were exposed to the cyclic salt fog test (1 h 5% NaCl exposure and 1 h 35 °C drying) for eight weeks and 10 panels were exposed to the cyclic salt fog test for 16 weeks. Rust creepage was not seen on any of the candidate coating systems; however, delamination was observed in one coating system after 16 weeks of exposure where Methods 1 and 2 were used for surface preparation. When surface preparation Method 3 was used with that coating system, delamination was not seen. According to the researchers, this indicates that surface preparation with grit blasting is critical for the performance of coating systems on Type 316 SS. On the test panels exposed for eight weeks, the coatings were stripped and the panels examined under the microscope for any signs of pit initiation. The results indicated that pitting was not found underneath the coatings for any of the candidate coating systems.

Only one coating system experienced delamination after 16 weeks of salt fog exposure (right). Panels with the same coating system did not experience delamination after eight weeks of exposure (left). Panels marked “S” were cleaned with Method 1, “F” were cleaned with Method 2, and “G” were cleaned with Method 3.

The researchers also tested the candidate coatings for holiday-free film formation, which measured the coatings’ ability to cover pits and determined if the coating system was holiday free before and after salt fog exposure. For the test, one coating system was applied to a panel by brush and the other nine coating systems were spray-applied. A wet sponge holiday tester detected holidays on all panels with spray-applied coatings, but no holidays were detected on the panel with the brush-applied coating.

For the nine spray-applied coating systems that failed the holiday test, new panels were coated—with primers brush-applied and the rest of coats spray-applied. Subsequent testing did not detect holidays on the new panels, which indicates that the primer must be brush-applied to adequately cover pits. The panels were then exposed to cyclic salt fog corrosion testing for four weeks. At the end of testing, holidays were detected for only one coating system.

At the end of the study, the researchers concluded that the most critical criteria for the selection of coating systems for offshore pitted SS are to have a good adhesion durability and low water permeation. They noted that coating systems behave differently on SS and CS. For most of the candidate coating systems applied to SS, underfilm corrosion or rust creepage was negligible, while various degrees of rust creepage is normally associated with these coating systems when applied to CS. The coating adhesion retention on SS was acceptable for most of the candidate coating systems tested, even when the surface was only solvent cleaned. However, to ensure the adhesion integrity on SS, the researchers prefer mechanical blast cleaning that creates a small surface profile (e.g., 1.5 mils). When coating pitted SS, the coating system selected should also form a holiday-free film. The researchers recommend applying the primer by brush to fill the pits and spray-applying the remaining coats.

As a result of this work, the researchers were able to identify three protective coating systems, as well as an application method, for the offshore SS equipment experiencing corrosion pitting. Out of the 10 coating systems tested, three passed all of the tests (Table 1) and are considered by the researchers to be qualified for coating offshore SS vessels and piping with corrosion pits.

References

1 T.T. Addis, et al., “Evaluation of Offshore Coating Systems on Pitted Stainless Steels,” CORROSION 2018, paper no. 10487 (Houston, TX: NACE, 2018).

2 ASTM G48-11 (2015), “Standard Test Methods for Pitting and Crevice Corrosion Resistance of Stainless Steels and Related Alloys by Use of Ferric Chloride Solution” (West Conshohocken, PA: ASTM International, 2015).

3 SSPC-SP 1, “Solvent Cleaning” (Pittsburgh, PA: Society for Protective Coatings).

4 NACE No. 2/SSPC-SP 10, “Near-White Metal Blast Cleaning” (Houston, TX: NACE International).

5 ASTM D4541-17, “Standard Test Method for Pull-Off Strength of Coatings Using Portable Adhesion Testers” (West Conshohocken, PA: ASTM International, 2017).

6 ASTM E96/E96M-16, “Standard Test Methods for Water Vapor Transmission of Materials” (West Conshohocken, PA: ASTM International, 2016).

7 ASTM G85-11, “Standard Practice for Modified Salt Spray (Fog) Testing” (West Conshohocken, PA: ASTM International, 2011).

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