Long term localised corrosion of marine steel piling welds

Abstract

Localised or pitting corrosion can be detrimental for steel pipes and containment structures, since wall perforation may cause system failure. Herein maximum pit depth quantification and its development with time are considered for samples taken from longitudinal welds on 33-year-old tubulars exposed in Newcastle Harbour. Relationships between pit depth and material metallurgy and corrosion properties were investigated by means of standard macro-etching, rest potential and zero resistance ammetry techniques. It is considered that the observed results are the result of the lack of homogeneity at the corrosion interface caused by differences in grain size, grain structure and the potential for pitting to occur preferentially along boundaries. The results are compared to measurements for longitudinal welds obtained previously on samples of API X56 Spec 5L pipe exposed in similar waters for up to 3?5 years, showing a reasonable degree of consistency between the two sets of data. The reasons for this are discussed.

Extreme value analysis for assessing structural reliability of welded offshore steel structures

“…The parameters considered include those related to corrosion, to pipeline dimension, to liquid flows and their respective related uncertainty, all treated as probabilistic (or random) variables. For serviceability the critical parameter is considered to be the loss of liquid through pit holes and for this pitting in the welding heat affected zone is critical in the analysis [5]. To assess the effect of the level of variability of the random variables on the failure probability a sensitivity study is also given for a typical example problem.…”

“…As described previously [15,18] extreme pit depths were obtained for each exposure period, including the 33 year samples, by physically measuring the depth of pits. By convention this is relative to a reference surface, and if there is general corrosion as well, making an allowance to obtain absolute rather than relative pit depths.…”

“…Figure 4. Trends of absolute maximum and mean pit depth readings and predicted power law depths for each weld zone [18] together with linear function approximations for the long-term rate of pit development.…”

“…This environment is recognized to be particularly aggressive. Moreover, the heat affect of the welding process in such structures is thought to cause increased localized pitting corrosion [5]. Structures of interest include harbour and coastal and offshore structures, ships and pipelines.…”

Long-Term Quantification of Structural Weld Capacity Loss Due to Localised Corrosion

“…where cs is the 'y-intercept' at time t = 0 and rs is the slope of the long-term trend. For the present study data from parent metal, weld and heat affected zones of mild steel welds exposed to Newcastle Harbour Australian Pacific waters [5,9] were used for the long-term quantification of maximum pit depth. This harbour is known to have relatively low levels of water pollution owing to high tidal flushing [10].…”