V ARIABLES INDEPENDIENTES

2.1 OBJECTIVES

Several aspects of low-pH SCC initiation and growth have been the subject of previous PRCI-funded projects. Despite this work, however, the relative SCC susceptibility of different pipeline steels has not been well documented. Recent EPRG-funded work has identified ways in which this issue can now be addressed.

Although the EPRG program has been extended to evaluate a wider range of materials, the testing is inherently slow and limited in extent. The objective of this program is to extend and enlarge this pool of data by conducting a parallel series of tests, using exactly the same laboratory conditions as those used by EPRG, on materials sourced from North America.

2.2 SCOPE OF WORK

The research program has been formulated based on the assumption that four suitable line pipe materials will be made available for testing, together with materials test certificates detailing mechanical properties (i.e. transverse and longitudinal yield and ultimate tensile strength) and chemical composition. The program costs assume that the line pipe sections required for this project will be supplied free-of-charge. Selection of the test materials will be finalised in conjunction with PRCI Committee Members.

The program is designed to evaluate low pH SCC initiation and early crack growth using standardised testing methodologies, under consistent, simulated field conditions and a fixed test duration.

2.2.1 TEST ENVIRONMENT

All tests will be carried out at ambient temperature with the specimen mounted in an air tight, sealed chamber containing NS4 solution (classically adopted as representative of the solution found in low pH SCC field studies. The solution will be pre-saturated at atmospheric pressure with a gas mixture of 90% N2 and 10% CO2.

Testing will be carried out at the free corrosion potential of the steel in NS4 solution for a duration of 90 days. All cyclic loading will be on the basis of a “sawtooth” cycle, with a 120 minute loading period and a 12 minute unloading period.

2.2.2 CRACK INITIATION TESTS

Test specimens will be removed from the pipe longitudinal axis and machined according to a modified (reduced thickness) ASTM E 8M specification for pin-loaded test specimens of 50 mm gauge length. The original uncoated (or coating-free surface) of the pipe material will be

preserved. All other machined surfaces will be coated to ensure that only the original pipe surface is in contact with the NS4 solution. Cyclic loading will be between 70% and 90% of the actual (measured) longitudinal yield strength of the pipe material.

• 2 tests per pipe material

2.2.3 CRACK PROPAGATION TESTS

Test specimens will be reduced thickness Compact Tension (CT) samples, based on ASTM E647 and prepared in the T-L orientation as defined in ASTM E399. All specimens will be fatigue pre-cracked to provide an overall crack depth of 14 mm. Cyclic loading with use a constant R value (Kmin/Kmax) of 0.78 with KImax selected above the likely threshold for low pH SCC (and inferred from previous EPRG test results).

• 1 test per pipe material

CONFIDENTIAL SCC INITIATION SUSCEPTABILITY RANKING/SCREENING (RPTG-0328) August 2002 Rev 0

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2.2.4 REFERENCE TESTS

Reference tests will be carried out on identical test specimens to those used in the crack

initiation and crack propagation studies. Two types of reference test will be carried out for each material included in the investigation:

1. Crack Initiation Reference – to be tested under identical environmental conditions with no applied stress.

• 1 test per pipe material.

2. Crack Propagation Reference – to be tested under cyclic conditions in laboratory air environment. These will be undertaken at a frequency of 0.1 Hz for 50000 cycles.

Details of the laboratory environment (i.e. ambient temperature and humidity) will be recorded throughout the test.

• 1 test per pipe material

2.2.5 TEST MONITORING

The following parameters will be monitored throughout the tests (excluding the control crack propagation tests):

• Specimen electrochemical potential – weekly

• Solution pH – fortnightly

• Solution conductivity – fortnightly

N.B. Previous experience from EPRG test programs suggests that the amount of crack growth is likely to be beyond the resolution of convential crack growth measurement technology (e.g.

clip gauges). It should therefore be recognised that the requirements of ASTM E647 cannot be met. In the case of the crack propagation tests, reliance is placed on the post-test examination of specimens to accurately determine the extent of crack growth.

2.2.6 SPECIMEN EVALUATION 2.2.6.1 Crack Initiation Specimens

The main assessment tool for evidence of crack initiation will be metallographic examination.

Following exposure of cyclic loaded and reference specimens, the complete gauge length of will be longitudinally sectioned through the mid-point of the specimen. The sections will then be polished and etched according to standard metallographic procedures.

The exposed pipe surface will be examined to determine the morphology and depth of surface features along the gauge length. To determine the probability of low pH SCC crack initiation, the features found on the surface of cyclic loaded specimens will be compared to sections through the reference test specimens and as-received pipe material.

2.2.6.2 Crack Propagation Specimens

To precisely distinguish the pre-crack, developed during the air pre-cracking phase from the crack growth generated during immersion in NS4 (and the fracture surface following mechanical rupture of the test specimen*), CT specimens will be submitted to the following heat-tinting procedure:

• Immersion of specimens in an acetone bath with ultrasonic treatment for 30 minutes

• Heat treatment in electric muffle furnace, in air, at 350oC for 1 hour followed by air cooling

* - The mechanical rupture of CT specimens will be carried out using a tensile machine, after immersion into liquid nitrogen

2.3 DELIVERABLES

The results of this program will be collated into a final report, which it is proposed will include (subject to agreement between EPRG and PRCI), a comparison of the data from previous EPRG test programs. This would considerably enhance the value of both EPRG and PRCI research programs, and increase our understanding of the metallurgical factors controlling low pH SCC crack initiation.

This research program will provide an increased understanding of the metallurgical parameters influencing the initiation and early growth of low pH SCC, and hence enable improved

management of SCC risks in existing pipelines, and better selection of materials for new pipeline construction.

The key deliverables from this programme will be as follows:

• Standardised and repeatable methodologies for the investigation of low pH crack initiation and propagation.

• Qualitative assessment and ranking of typical North American line pipe materials to low pH SCC crack initiation

• Comparative data on the resistance to low pH SCC of typical North American and European line pipe materials.

CONFIDENTIAL SCC INITIATION SUSCEPTABILITY RANKING/SCREENING (RPTG-0328) August 2002 Rev 0

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2.4 SCHEDULE

The work will be undertaken by Advantica over a 21 month period. A summary of the main tasks of the project and their duration, assuming work commences in Q1 of 2003, is detailed below. Effort requirements are also provide for each task.

2003 2004 PHASE

Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 1 Preparation ^Specimen

0.4 man.mths

Phase 1 – Specimen Preparation Duration – Year 1: months 1 to 3

• Delivery of pipe lengths

• Handling and storage of pipe lengths

• Flame cutting of pipe sections for test specimen stock

• Machining of test specimens

• Delivery of test specimens Phase 2a – Testing

Duration – Year 1: months 4 to 7

• Crack initiation and reference tests: steel No.1 Phase 2b – Testing

Duration – Year 1: months 7 to 10

• Crack initiation and reference tests: steel No.2 Phase 2c – Testing

Duration – Year 1: months 10 to Year 2: month 2

• Crack initiation and reference tests: steel No.3

• Crack propagation & reference tests: steel Nos.1 and 2 Phase 2d – Testing

Duration – Year 2: months 3 to 7

• Crack initiation and reference tests: steel No.4

• Crack propagation & reference tests: steel Nos.3 and 4 Phase 3 – Final Report

Duration – Year 2: months 8 to 9

• Preparation of Final Report