Appendix – Test Procedures for Qualification of Corrosion Resistant Steel for Cargo Tanks in Crude Oil Tankers

  2.1.1 Test condition

Tests on simulated upper deck conditions in cargo oil tank (COT) shall satisfy each of the following conditions:

• .1 Corrosion resistant steel and conventional steel shall be tested at the same time.

• .2 The chemical composition of conventional steel shall comply with the requirements of table 1. The mechanical properties of the test specimen should be representative of steel used in its intended shipboard application.

• .3 The tests for corrosion resistant steel shall be carried out for 21, 49, 77 and 98 days. The tests for conventional steel shall be carried out for 98 days. The tests for welded joints shall be carried out for 98 days.

• .4 There are to be five test pieces for each test period.

• .5 The size of each test piece is 25 ± 1 mm x 60 ± 1 mm x 5 ± 0.5 mm. The surface of the test piece shall be polished with an emery paper #600. The size of the test piece for a welded joint is 25 ± 1 mm x 60 ± 1 mm x 5 ± 0.5 mm, including 15 ± 5 mm width of the weld metal part.

• .6 The surface of the test piece, except for the tested surface, shall be protected from corrosive environment in order not to affect the test results.

• .7 The test apparatus consists of a double chamber, and the temperature of the outer chamber is to be controlled.

• .8 Simulating the condition of the actual upper deck, the test cycle runs with distilled water and simulated COT gas (4 ± 1% O₂ - 13 ± 2% CO₂ - 100 ± 10 ppm SO₂ - 500 ± 50 ppm H₂S - 83 ± 2% N₂). A sufficient distance between the surface of the test piece and the distilled water is to be kept to avoid splashing of distilled water. The minimum gas flow rate is 100 cc per minute for the first 24 h and 20 cc per minute after 24 h.

• .9 The test pieces shall be heated for 19 ± 2 h at 50 ± 2°C and 3 ± 2 h at 25 ± 2°C and the transition time is to be at least 1 h. The time for 1 cycle is 24 h. The temperature of the distilled water is to be kept at not higher than 36°C, while the temperature of the test pieces is 50°C.

  2.1.2 Test results of base metal

 Prior to the testing, the following measured data shall be reported:

• .1 size and weight of the test piece;

 and, after the testing, the following measured data shall be reported:

• .2 weight loss (difference between initial weight and weight after testing) of conventional steel (W_C) and corrosion resistant steel (W₂₁, W₄₉, W₇₇ and W₉₈);

• .3 corrosion loss of conventional steel (CL_C) and corrosion resistant steel (CL₂₁, CL₄₉, CL₇₇ and CL₉₈), calculated by the following formulae:

whereby:

• W_c : weight loss of conventional steel (g) (average of five test pieces)
• W₂₁ : weight loss of corrosion resistant steel after 21 days (g) (average of five test pieces)
• W₄₉ : weight loss of corrosion resistant steel after 49 days (g) (average of five test pieces)
• W₇₇ : weight loss of corrosion resistant steel after 77 days (g) (average of five test pieces)
• W₉₈ : weight loss of corrosion resistant steel after 98 days (g) (average of five test pieces)
• S : surface area (cm²)
• D : density (g/cm³).

• The test is considered to be carried out appropriately if CL_C is between 0.05 and 0.11 (corrosion rate is between 0.2 and 0.4 mm/year). The concentration of H₂S in simulated COT gas may be increased for adjusting CLC;

• .4 coefficients A and B of corrosion resistant steel, calculated from the test results for 21, 49, 77 and 98 days by least square method.

• Corrosion loss of corrosion resistant steel is described as follows:

  

  A(mm) and B: coefficient

  t: test period(days);

• .5 estimated corrosion loss after 25 years (ECL) calculated by the following formula:

  

  2.1.3 Test results of welded joint

 The surface boundary between base metal and weld metal shall be observed by microscope at 1,000 times magnification.

  2.1.4 Acceptance criteria

 The test results based on provisions of 2.1.2 and 2.1.3 shall satisfy the following criteria:

• .1 ECL (mm) ≤ 2 (for base metal); and

• .2 no discontinuous surface (e.g., step) between the base metal and weld metal (for welded joint).

  2.1.5 Test report

 The test report shall include the following information:

• .1 name of the manufacturer;

• .2 date of tests;

• .3 chemical composition and corrosion resistant process of steel;

• .4 test results according to 2.1.2 and 2.1.3; and

• .5 judgement according to 2.1.4.

  2.2.1 Test condition

 Tests on simulated inner bottom conditions in cargo oil tanks (COT) should satisfy each of the following conditions:

• .1 The test shall be carried out for 72 h for base metal, and 168 h for welded joint.

• .2 There are to be at least five test pieces of corrosion resistant steel for base metal and welded joint, respectively. For comparison, at least five test pieces of base metal of conventional steel should be tested in the same condition.

• .3 The size of each test piece is 25 ± 1 mm x 60 ± 1 mm x 5 ± 0.5 mm for a specimen with base metal only, and is 25 ± 1 mm x 60 ± 1 mm x 5 ± 0.5 mm for a specimen with welded joint including 15 ± 5 mm width of weld metal part as shown in figure 3. The surface of the test pieces shall be polished with an emery paper #600, except a hole for hanging.

• .4 The samples are hung in a solution from a fishing line (0.3 mm to 0.4 mm in diameter, made of nylon) to avoid crevice-like and/or localized corrosion. An example of a corrosion test configuration is shown in figure 4.

• .5 The test solution contains 10 mass% NaCl and its pH is 0.85 adjusted by HCl solution. The test solution should be changed to a new one every 24 h to minimize pH change of the test solution. The volume of the solution is more than 20 cc/cm² (surface area of test piece). The temperature of the test solution is to be kept at 30 ± 2°C.

  2.2.2 Test results of base metal

 Prior to the testing, the following data shall be measured and reported:

• .1 size and weight of test piece;

 and, after the testing, the following measured data shall be reported:

• .2 weight loss (difference between initial weight and weight after testing);

• .3 corrosion rate (C.R.) calculated by the following formula:

whereby:

• W: weight loss(g), S: surface area(cm²), D: density(g/cm³);

• 4 to identify specimen which hold crevice and/or localized corrosion, the C.R. is to be plotted on a normal distribution statistic chart. C.R. data which deviate from the normal statistical distribution must be eliminated from the test results. An example is shown in figure 5 for reference;

• .5 calculation of average of C.R.'s data (C.R. _ave):

  2.2.3 Test results of welded joint

 The surface boundary between base metal and weld metal shall be observed by microscope at 1,000 times magnification.

  2.2.4 Acceptance criterion

 The test results based on sections 2.2.2 and 2.2.3 shall satisfy the following criteria:

• .1 C.R. _ave (mm/ year) ≤ 1.0 (for base metal); and

• .2 no discontinuous surface (e.g., step) between the base metal and weld metal (for welded joint).

  2.2.5 Test report

 The test report shall include the following information:

• .1 name of the manufacturer;

• .2 date of tests;

• .3 chemical composition and corrosion resistant process of steel;

• .4 test results according to 2.2.2 and 2.2.3; and

• .5 judgement according to 2.2.4.