Dear All,
I have been involved in a project regarding to estimation of remaining service life of corroded galvanized structural elements. how can i find the remaining service life based on lab tests and real situation of the corroded elements? I have an environmental chamber and an salt spray chamber is it possible for put some uncorrected galvanized steel in the salt spray chamber and measure the corrosion over some certain hours like 24, 48, 96, 128, 256,500, 1000.then find the rate and calibrate it with the years that the member has serviced and the amount of corrosion. then find the remaining lifetime based on remaining thickness?
cheers
Dear Nariman
I hope to tell more about:
How long the structure was in service yet
The approximate rusting percent of the steel surface (if any)
Type of environment , rural, marine, industrial ,ect.
Average thickness of zinc coating
this will help to go for farther discussion.
Best regards
Dear Dr. Nariman Saeed,
what you propose can be an approximate evaluation of the service time of the incorrect elements. In reality, the elements will go into a more complex system that could generate different chemical-physical factors that can significantly change this data. In these tests it is also assumed that all the incorrect elements have the same defects and almost similar behaviour, otherwise you should provide a number (at least 3) of replicas to work on. Naturally, before and at the end of each test, a digital photographic documentation and a characterization of the corrosive phenomenon (also from a morphological point of view, especially if there is evidence of localized corrosion) is, in my opinion, necessary.
Best regards,
Pierluigi Traverso
Dear Dr. Nariman Saeed;
I consider that determining the remaining life time of a structural element depends on the type of corrosive phenomenon present, and on the stresses to which the element is subjected; that is to say:
You can not treat an element subject to internal pressure (tanks, boilers or pipes), in the same way as one subjected to torsion or compression.
In the case of tanks, the API design standards consider an over thickness of corrosion, and safety factors to prevent the plates from reaching the yield stress. Therefore, if (despite the protection measures) due to the corrosion effect, the excess corrosion thickness has already been lost, the remaining thickness affects the value of the stress that tends to come closer and closer to the yield stress, therefore - a criterion, it may be to consider that the element has lost security and therefore must be changed, if the loss of thickness - exceeds the over thickness of corrosion.
A similar criterion can be considered for structural elements subjected to generalized atmospheric corrosion, subjected to tensile stress. In the case of elements subjected to compression, the risk of buckling or loss of rigidity should be determined, with the loss of thickness.
However, the attack by corrosion is not always generalized, for which the over-thickness of corrosion is thought; and the localized attack - by pitting or the presence of intergranular corrosion or cracks, can bring into consideration a change that can be considered premature, since the loss of mass (and thickness) can be minimal, however the presence of pits or cracks, and the extension - can force the change of the partial or total element, or if it is very punctual, consider if it is possible to apply a reinforcement - trying to return the structural integrity. In the case of pipes, there are even ANSI / ASME B31 G standards that allow estimating the magnitude of the remaining stress, based on the geometry and depth of the pits.
There are also other phenomena (creep by temperature) that are not directly linked to electrolytic corrosion (boiler and furnace pipes), and that can generate intergranular defects and the appearance of cracks, where there is no loss of mass (thickness) and instead the change of the element is required, before the failure occurs.
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