Description
P 918 – Proof of equivalence of novel corrosion protection coatings for steel crash barriers
Road restraint systems are crucial for traffic safety on roads. Of course, the longevity of products is fundamental to maintain the safety function, therefore a lifetime of at least 25 years is being demanded. For steel guardrails the corrosion protection plays a major role with regard to that. Following the specifications given in Germany by the Technischen Lieferbedingungen für Stahlschutzplanken (TL-SP 99) [5], these parts shall be manufactured from steel by profiling and subsequent hot dip galvanization, according to DIN EN ISO 1461 [6].
Using new corrosion protection coatings according to DIN EN 10346 [7] (continuously hot-dip coated steel products) or DIN 50997 [8] (Zinc-aluminium coatings applied by thin film galvanizing) would offer a number of immediate advantages due to the different coatings or production process, such as:
- Sustainability advantages (resource efficiency, durability, etc.),
- Economic advantages and also
- Safety relevant advantages.
Within the scope of this research project the following corrosion protection coatings for steel guardrails have been investigated and compared with regard to their equality:
- Z 275 – continuously hot-dip coated according to DIN EN 10346 [11];
- Z 600 – continuously hot-dip coated according to [11];
- ZA 300 – continuously hot-dip coated according to [11];
- ZM 140 – continuously hot-dip coated according to [11];
- ZM 300 / ZM 310 – continuously hot-dip coated according to [11];
- ZM 430 – continuously hot-dip coated according to [11];
- ZnAl5-dip – thin film galvanized according to DIN 50997 [12]
- hot dip galvanized according to DIN EN ISO 1461 [10] (reference)
Natural weathering in-field applications have been installed on two selected highway sections (BAB45 and BAB61) and being evaluated over a period of five years. The highway sections have been selected in particular in view of a comparably high average winter service, i.e. rather high use of salt grit, because of the comparably high contamination of corrosion attack, particularly with regard to the guardrail posts with their contact to the surrounding soil.
The results from magnet inductive coating thickness measurements after five years infield natural weathering show no significant reduction for none of the different corrosion protection coatings.
In view of further experiences also know from literature it can be assumed, that there will be probably no relevant reduction of protection coating thickness even for longer periods.
For none of the unprotected (longitudinal) cutting edges of the guardrail beams any red rust could be observed after five years natural weathering; for the unprotected cutting edges of the bolt holes or for the cross section some beginning red rust could be observed. Also, it could be observed that the number of local spots of flash-rust are increasing.
On the other hand, the unprotected cutting edges of the Z-continuously coated guardrail posts (both longitudinal as well as cross cutting) have shown red rust already as early as after one year. Much less but also noticeable this has been observed for the ZMcontinuously coated guardrail posts. Due to the rather high material thickness of the posts (up to 5 mm) the cathodic protection seems to be not enough, even for the ZMcontinuously coating. However no evidence for any kind of subsurface corrosion starting from those edges could be observed. Also no additional rust development with potential bolt hole widening could be observed at the bolt holes. Although the metallographic laboratory testing showed very local exposure of the substrate material for the Z-continuously coating, no posts developed any kind of plane red rust apart from the edges and apart from some local spots of flash-rust.
Not like under the atmospheric environment the posts, when in contact to the soil, show some degradation of the coating thickness, which should be further investigated. At this time some posts have been pulled out (and exchanged by some other posts) after one and after three years in-field, so that the lower parts of those posts could be investigated. The degradation measured using the method of gravimetry, gave already after one year for Z-coated posts taken from the BAB61 site a peak-value of 14.5 µm.
The average value has been 7.0 µm/year; for the other site at BAB45 the measured degradation has been generally less with an average value of 4.7 µm/year. Also, it has been observed that the ZM-coatings seem to be much less effected by the soil surrounding: For those ZM-coatings the degradation has not been significantly. The ZA-coatings as well, seem to be less effected. However, due to the particular thin thickness of the ZA-coatings it may be recommended most for them to be observed further.
Besides those extensive investigations on those steel guardrails that have been out in the field subjected to natural weathering there have been also a number of laboratory tests according to VDA 233-102:
Due to the more severe exposition all specimens in the VDA alternating climate chamber tests show red rust not only on the edges, but on the steel planes, but at different number of cycles/ weeks. For those thin coatings ZnAl5-dip and ZM140 this started already to develop widely on the specimens after three weeks, whereas the specimens with a ZA300 or Z275 coating at that stage developed only local red rust planes. When extending the duration of testing to six weeks the corrosion widens out, with now also specimens with ZM310 or Z600 coating showing first red rust on the uncovered edges. Finally, after 12 weeks testing all specimens of all coatings have developed red rust on the steel planes.
Taking metallographic views of all specimens after those 12 week cycles, it can be stated that coatings Z600, ZM300 and ZM310 have been performing similar to the reference hot-dip galvanized material, while all the other investigated coatings have been showing much stronger corrosion.
Looking at the bolt hole edges, it can be seen for all specimens (except the reference material) that rust appears soon, however no specimen shows even after 12 weeks in the climate chamber any kind of bolt hole widening.
Considering the much more severe corrosion on the investigated specimens during the laboratory testing in the alternating climate chamber in comparison to the real in-field orrosion, it seems that the specifications of the laboratory tests should be modified further: In particular the heavy occurrence of white rust, which hardly any is seen infield seems to advise to lower the humidity phases. Also, the concentration of NaCl seems to be still too high. Therefore, one has to infer that it is not possible to correlate the outcome of the laboratory tests with regard to a prediction of the long-time durability of such specimens of steel guardrails in real atmosphere in-field.
As a conclusion, it seems that clearly the thicker coatings of ZM300 to ZM450 show both in-field (guardrail beams in atmosphere and also guardrail posts in soil) as well as during the laboratory tests a very comparable protecting level than the reference hotdip galvanized material.
Although the thinner continuously galvanized ZM140-, ZA- and thinner hot-dip ZnAl5-dip-coatings clearly show earlier red rust within the laboratory tests, however, since there is no correlation from the outcome of the laboratory tests to predict long-time durability, this performance should not lead to an exclusion of those coatings, especially since at this point after five years natural weathering there is hardly any reduction noticeable of the coating thickness for the guardrail beams in atmosphere.
On the other hand, the influence of the surrounding soil condition should be observed further on, before a safe recommendation can be given to use these new alternative coatings. In particular the usage of Z275 and Z600 coatings seem to be less advisable for posts, rammed into soil.
All research reports in german language!
Authors:
M. Sc. Ch. Blankart, Univ.-Prof. Dr.-Ing. U. Krupp, Dr.-Ing. G. R. Heßling, Dipl.-Ing. A. Geßler, Univ.-Prof. Dr.-Ing. M. Feldmann
