UV-curable varnishes are used to protect pipes against corrosion during transport and storage prior to their final use (i.e. temporary protection). A positive aspect of these varnishes is that they are environmentally-friendly because volatile organic compounds (VOC’s) are not released during their application.

In-house qualification of varnishes for temporary protection currently involves 3-6 months outdoor exposure tests at selected geographical sites in particular at a seaside site classified C4 according to ISO 12944-2 (Dunkirk). However requirement of a good pipe appearance for increasingly longer periods of time has lead to a continuous demand and offer of new UV-curable varnishes and a corresponding need for longer-lasting outdoor exposure tests.

It would be convenient to be able to assess new varnishes by means of short-term laboratory tests. Accordingly neutral salt spray tests such as those described in ISO 9227 have been tried but proven not representative of outdoor exposure site(s) conditions.

Thus it has been set as an objective to develop an accelerated corrosion test in which UV-curable varnishes will exhibit similar degradation (modes and extent) to that observed following exposure at selected outdoor sites for 3 6 and 12 months. Here progress made to date towards achievement of the latter objective is presented.

As a first step the chosen outdoor exposure sites were characterised according to ISO 9223 standard. Consistently the relative humidity temperature and main pollutants (SO2 NOx and Chlorides) were monitored for up to 12 months in each site. Also standard coupons and varnished tube samples were exposed to assess the corrosion rate and degradation morphology respectively.

Based on the input from the sites characterisation known accelerated weathering tests were pre-selected in order to compare the degradation obtained following exposure outdoor and that in each of the tests conditions. The pre-selected tests were the neutral salt spray ISO 20340 and VDA 621-415B. The exposure times were initially chosen based on previous experience refinement was then possible upon experimental observations.

The results from visual examination (according to ISO 4628-3) revealed that a correlation can be made between the morphology of degradation after exposure at Dunkirk and after exposure to the VDA test for specific periods of time.

Analysis by Modulated Differential Scanning Calorimetry (MDSC) Fourier Transform Infrared (FT-IR) Spectrometry and Electrochemical Impedance Spectroscopy (EIS) allowed determination of the changes in the physicochemical properties of the varnishes with time after exposure to the different environments. In particular EIS in 5%wt NaCl was used to:
- better qualify varnishes (correlation to natural exposure)
- define additional acceptance criteria
- understand degradation mechanisms in order to better explain varnishes performance

It has been observed that the criteria |Z| (at 0.001Hz) ? 106 ? cm2 can be used as an acceptance criterion to have sufficient barrier property associated with expected outdoor protection. Furthermore a fairly good relation was also found between the results from EIS and the VDA test results.