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Solving Technical Challenges to Test in De-aerated Seawater

TWI has been asked by several Industrial Members to perform mechanical tests in de-aerated brine solutions under Cathodic Protection (CP), i.e. in a 3.5% NaCl solution, at 4°C and applying a potential of -1100mV vs. Ag/AgCl. This arrangement is considered to provide conditions that are representative of those experienced by some subsea components and systems under CP. In impressed current CP systems (ICCP), gas evolution through electrolysis of brine can lead to formation of chemically aggressive species (in particular chlorine) that may lead to degradation of structural steels. The severity of the development of diffusible hydrogen in the steel can also be different in aerated and de-aerated conditions due to different cathodic reactions, and therefore testing in representative service conditions is important to generate relevant fracture and fatigue data for assessing the integrity of subsea components.

The challenge is in developing a representative de-aerated seawater test condition in the laboratory that maintains a stable pH for the full duration of environmental tests (often several weeks), while minimising the production of toxic and corrosive chlorine and by-products thereof. In service, any chlorine gas evolved is diluted within the large volume of the sea, but in the laboratory the smaller scale can mean the accumulation of chlorine to a more significant degree. The need to completely exclude dissolved oxygen from the solution also poses practical challenges. Risk of ingress of air can be mitigated by creating a fully sealed test environment, but additional measures are also required as oxygen can be also be generated in-situ during electrolysis.

Reducing the level of dissolved oxygen (DO)

De-aeration of the 3.5% NaCl solution was done by a series of purges with nitrogen gas. TWI has established a procedure to enable a dissolved oxygen level of below 10ppb (parts per billion) to be achieved, and the de-aerated seawater test vessel was adapted in order to ensure <10ppb could be maintained throughout the test.

Material selection for equipment used in testing with de-aerated aggressive environments is challenging. The suitability of the materials used in any test configuration should be carefully considered at the outset of any project. In the current case, tests were carried out using specialised low permeability polymer vessels, which are both resistant to chlorine and have low oxygen permeability. The testing jigs and loading frames need to be from an alloy which is resistant to corrosion and the effects of chlorine gas. Titanium offers the best performance under de-aerated test conditions, but is significantly more expensive than stainless steels. TWI has had success performing testing using equipment made from polished 316L stainless steel with low surface roughness and where the load frame is not exposed directly to chlorine.

In order to address the issue of in-situ oxygen and other oxidizing species (i.e. chlorine and hypochlorite) produced at the anode, a semi-permeable ceramic barrier was used to isolate and localise these species. This process promoted removal of gaseous by-products formed at the anode to be bubbled off safely to extraction, while minimising the risk of transport of these corrosive species to the cathode (i.e. the steel test sample). An additional advantage of this approach is that it promotes uniform current distribution from the anode to cathode and avoids the risk of excessively large cathodic potentials developing on the test sample surface closest to the anode. This approach is routinely employed in marine ICCP systems.

The presence or formation of any solution contamination can compromise both the test environment and the reliability of the tests, and should be avoided. TWI uses specific cleaning procedures for test equipment for performing tests in potentially corrosive chemical environments. The surface condition of the equipment prior and after testing should also be checked and, where required, remediation or replacement of damaged items should be mandated.

Dissolved oxygen is continuously monitored in-situ during testing using a calibrated Orbisphere instrument and sensor, while the chlorine content was also measured at regular intervals using dip test strips.

Maintaining a stable test environment

It is important to note that if tests were carried out within a single sealed vessel, the chlorine formed is likely to lead to a reduction in solution pH over time (i.e. increased acidity), which is undesirable in an environmental test that requires control and maintenance of stable conditions over extended periods of time.

Use of the semi-permeable ceramic barrier at the counter electrode (i.e. impressed current anode) provides the means to minimise gross contamination of the test solution, while still allowing solution recirculation. This is one of the major challenges to performing effective mechanical tests in de-aerated seawater, and TWI has carried out a number of trials in order to optimise the method it now uses.

Electrolysis of the test solution results in a slow but progressive change in the solution composition and pH. In order to address the latter, which is often more important, continuous pH control of the test solution is managed by using an automatic acid-injection system with feedback control, and membranes around the specimen. The process of automatic monitoring and adjustment of the pH ensures stability of test environment for the duration of the loading and thus the quality of the test data.

In order to avoid significant changes to the composition of the test solution, through consumption of reagents and production of reaction products, a large solution volume to specimen surface ratio is normally desired for environmental tests. Although this ratio is taken into consideration at the design stage of offshore/marine components, there may be no clearly defined and evident criteria/guidelines for carrying out lab scale tests. This is an area where there is a need for the development of guidelines.

Figure 1. Schematic drawing of the configuration for testing in de-aerated seawater, where:
RE = Reference electrode, which allows measurement of the voltage during testing.
CE = Counter electrodes, from which the impressed current for the cathodic protection is applied 
WE = Working electrode, which is the steel specimen under test
Figure 1. Schematic drawing of the configuration for testing in de-aerated seawater, where: RE = Reference electrode, which allows measurement of the voltage during testing. CE = Counter electrodes, from which the impressed current for the cathodic protection is applied WE = Working electrode, which is the steel specimen under test

Mechanical testing in de-aerated seawater

With a stable test environment including CP in place, it is possible to carry out mechanical tests such as slow strain rate tensile (SSRT), fracture toughness and fatigue crack growth rate tests on steel specimens. The absence of oxygen (i.e. a readily available cathodic reactant) in de-aerated systems typically means that higher applied current values are required in order to achieve a similar level of CP in de-aerated tests relative to conventional seawater tests in air. Specimens are often pre-charged in-situ (without applied loading) for a number of days before the test commences (depending on the type of specimen being tested).

Following from TWI’s development of the test capability, equivalent testing on specimens in both aerated and de-aerated seawater conditions should be performed to fully quantify the influence of dissolved oxygen on subsea performance of steels.

Summary

Ensuring the long-term stability of the test environment when characterising materials properties in de-aerated seawater under ICCP poses significant challenges. Additional safety concerns are created due to decomposition of the test solution through electrolysis, resulting in formation of toxic chlorine and oxygen contaminant (both directly at the anode and indirectly through breakdown of ensuing hypochlorous species). For further development of this testing, potential interactions between CP systems and DCPD crack growth monitoring methods should be considered, along with the efficiency of applying coatings as a methodology to increase the solution volume to specimen surface ratio where there could be a risk of disbonding under CP. Use of galvanic anodes (for which current density values are significantly lower than for ICCP anodes) to provide CP is currently being investigated as a practical solution for the avoidance of chlorine gas production during tests

Despite the technical challenges raised during these tests, TWI has successfully developed new capability and expertise in fully controlled environmental testing in de-aerated seawater under ICCP and galvanic anode CP for tensile, fracture and fatigue tests.

Figure 2. De-aerated seawater test equipment in TWI laboratory
Figure 2. De-aerated seawater test equipment in TWI laboratory
Avatar Marie-Eleni Mitzithra Senior Project Leader Technology

Marie-Eleni joined TWI on October 2014 as a Corrosion Engineer. Principal studies were carried out on electrochemistry and materials. Marie-Eleni's PhD thesis was carried out on the detection of corrosion of steel reinforcement embedded in concrete. At TWI Marie-Eleni has been working on projects related to the corrosion performance of ferrous and non ferrous alloys (Al, claddings etc) ie failure investigations, electrochemical and exposure testing of materials in different environments (ie sweet, sour, seawater), etc.

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