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.