Problem
The corrosion protection of steel structures can be found in most cases by applying a protective layer. Maintenance on conservation is expensive due to the accessibility facilities. Therefore the aim is to create a large potential expected life span. There are several types of preservation possible: a conventional protective paint system, hot-dip galvanizing and a metallic sprayed coating. Hot-dip galvanizing is not suitable for all parts, because the zinc bath dimensions are limited and there are risks on deformation of the structure. With a conventional protective paint system the life span is limited, especially coatings used on an immersed structures have a maximum life of 20 years. A possible solution is applying a thermal sprayed coating in combination with an organic coating. The advantages are first the Anodic process after damaged has occurred and secondly the hard layer. The expected life span of such a system is 40 to 50 years. Often structures have to deal with fatigue loads. Therefore these structures are NDT examined regularly. The question is whether the current NDT techniques are suitable for metallic sprayed layers.
Approach
There are already several structures at Rijkswaterstaat equipped with a thermal coating, such as the lock gates of lock Kats, lock Hollandsche IJssel, a rolling gate at Terneuzen and the bridges of Goeree. However, the discussion about fatigue sensitive structures started whether fatigue cracks may or may not be detected during the use phase. For the structural safety, it is important to be able to continue to monitor the structure on (fatigue sensitive) cracks. In 2013 TNO has carried out an investigation in which different types of thermal sprayed metals on steel test pieces are loaded to a visual fatigue crack length of 20 mm. Various NDT techniques are applied.
Product
TNO has the report “Effect of thermal sprayed coating Al or Zn/Al 85/15 on crack detection steel structures”. It is shown that the common NDT techniques: penetrant examination, ultrasonic technique and Eddy current technique give no usable information at cracking. The conclusion is that no cracking as a result of fatigue load can be detected.
Benefits
TNO has the report “Effect of thermal sprayed coating Al or Zn/Al 85/15 on crack detection steel structures”. It is shown that the common NDT techniques: penetrant examination, ultrasonic technique and Eddy current technique give no usable information at cracking. The conclusion is that no cracking as a result of fatigue load can be detected.
Bedrijf: Rijkswaterstaat / TNO
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