Corrosion prevention in the hydro industry23 May 2014
Anders Braekke from Jotun gives his thoughts on corrosion protection in the hydro industry.
Effective corrosion systems and good maintenance programmes are important factors to reduce down time, maximise output and secure a long life for hydro installations.
Jotun's core competence has been in heavy corrosion protection since 1930. This has been mainly in the hydro, offshore and land based industries. What we see in the hydro industry is that atmospheric conditions are different with different challenges. Don't get fooled - fresh water is a bigger challenge than the salty sea water.
One of the problems in a fresh water environment is osmosis. The cleaner the water the larger the risk of blisters and coating failure. The surface salt level is especially critical during new construction. By coating systems with a high osmosis tolerance we have witnessed the value of highly tolerant epoxy coatings with a high solid content over the past 30 years
Salt - the biggest challenge in fresh water
We recommend to have a maximum salt level of 20mg/m2. This is actually identical to NORSOK new build in offshore specifications. Even higher salt levels in a traditional offshore environment can be accepted, but this is in a salt water environment only. In hydropower plants we are careful to measure the surface salt level and have this as low as possible. Areas like the penstock and gates are different from offshore installations since they do not have cathode protection. The full responsibility for a corrosion free surface is put on the coating alone. The cathodic protection for other types of installations can be seen as an extra safety net if the coating is failing. When we recommend a maximum of 20mg/m2 - the same level as we accept in fresh water tanks - this will, together with a good epoxy coating, secure a long lasting, trouble free protection.
There is a risk of osmosis for all immersed areas in fresh water. Jotun's technical advice is to follow ISO12944 standard and focus on products with the best quality and not to look at generic type alone. We still recommend using a high quality epoxy mastic. Our experience is that high solid epoxy mastic gives the best protection in 500 micron/20 mils dry film thickness in two coats. This gives full protection even against osmosis if pre-treatment is correct. The issue with osmosis in freshwater is well known and ISO is now evaluating to increase the dry film thickness even further.
The Jotamastic 87 has been Jotun's top seller in the hydro industry for many years. It has a quality and core technology that fits well to the challenges seen in hydro industry. A multipurpose product, it is used in new construction, refurbishment and maintenance situations.
Epoxy mastics are specially designed to be all-round, surface tolerant coatings. The reasons for the very good penetrating properties are the small sized molecules and the low viscosity of the binder, which gives good flow. As such they can be used with very good results on hand and power tool cleaned surfaces, water jetted surfaces, Magnesium descaled surfaces (electrolytic descaling) and on blast cleaned surfaces. In summary this gives epoxy mastics added flexibility to the owner/operator with regards to where and when to do the maintenance.
The diagram shows different size molecules 'flowing' into the profile of the steel substrate, or in the case of the larger Epoxy molecules they are 'sitting' on the surface. The hydrocarbon resins are small and allow the epoxy mastics to penetrate deep and create optimised adhesion on reduced surface preparation.
Challenges for hydro
In many countries the zebra mussel invasion is a real threat to marine life and installations. This invasive species came from Russia around 1980 and is now found in many lakes worldwide. In the US it is soon expected to reach the Mississippi river system.
Different technologies (anti fouling, Zequanox or pulse technology) may reduce the problem, but the challenge is then to avoid affecting other species. Products with some easy slip effect are most realistic, with the objective to have easy manual cleaning for gates and penstock. Most of the technologies have long term experience from the shipping industry but more research is needed to be able to control the zebra mussel.
Jotun's policy will be to continue promoting epoxy mastic for the hydro industry. Any coating for mussel prevention will be applied directly onto the epoxy mastic, as the third coat. This will ensure that we maintain excellent corrosion protection and reduce the problems with fouling and mussels.
Anders Braekke is the Global Concept Director for Energy Performance Coatings at Jotun A/S.