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Qualification of materials for use in drinking water applications: Test method for stainless steel
(2007)
Elektrochemische Messungen zur hygienischen Bewertung additiv gefertigter Bauteile werden diskutiert. Die Bewertung und Details des Werkstoffs werden beschrieben, Anwendungsbeispiele gezeigt.
Corrosion of copper alloy fittings in a combined plastic/metal installation for hot drinking water
(2014)
A household installation for drinking water was analysed for the water quality after an incident concerning the water distribution system in the city. Chemical analysis showed very high levels of lead in the hot water circulation system. The installation was made from multi-layer-pipes, the hot water tank from stainless steel, no obvious source of lead could be determined. After a thorough cleaning of the piping system and the tank the lead levels rose again after short time.
- INVESTIGATIONS:
Water samples and samples of the pipe connectors made from different copper alloys were taken during an inspection of the building. Results for water analysis and metallographic investigations of the samples are given. Close examination of the connectors showed selective corrosion along the grain boundaries for the connectors made from brass but not for the connectors made from gun metal.
- RESULTS:
The lead found in the hot drinking water war released from the brass connectors. Obviously the contact time of the water with the metal surfaces in the system investigated was much longer as the contact times used for the evaluation of metallic materials according to EN 15664-1. The standards for the dimensioning of hot water circulation systems available are outdated and do not consider retention time in the system but are driven by maximum availability of hot water at any time.
A new Installation for hot drinking water showed high lead concentrations in the water. The installation was constructed from plastic pipes, the hot water tank from stainless steel. A source for the lead contamination was not obvious, cleaning of the tank and the pipes did result in low lead levels after the cleaning but these rose again after short Operation times.
Water samples and samples from the installation were taken and analysed. The high lead values for hot water circulation were confirmed, the cold water System showed no detectable lead values. Metallographie investigations showed selective corrosion of various pipe fittings from dezincification resistant brass.
The cause of the lead contamination was the selective corrosion of brass combined with a long retention time of the hot water in the circulation System.
Five stainless steel alloys were tested and compared to find a test method for the qualification of stainless steel used in drinking water systems. Determination of the pitting/critical potentials from the conventional cyclic polarization results was shown to be difficult when stainless steels have high Mo and Cr content. The electrochemical impedance data indicated that results received with the conventional methods do not necessarily reflect the steady state. The low frequency time constant determined from the impedance measurements showed that the anodic polarization should be conducted with very slow scan rate to get information about the mechanism. The steady state investigations were compared with the conventional electrochemical results to propose an electrochemical-based route as a standard qualification method. The qualification method was tested to be valid by a European round-robin-test.
The kinetics of the anodic dissolution of brass (CuZn42 and CuZn21Si3P) in synthetic tap water were investigated by atomic emission spectroelectrochemistry. Elemental Cu and Zn dissolution rates were measured in situ and in real time during galvanostatic dissolution. A complete mass/charge balance for the system yielded, as a function of applied current and a function of time, the quantity of Cu in the dezincification layer and the quantity of Cu and Zn in the oxide layer. In this way, a complete kinetic characterization of the fundamental chemical processes occurring during dezincification was realized for the first time. The oxide layer was composed primarily of Cu2O as indicated by grazing incidence XRD and Raman analysis. The soluble Cu oxidation product was determined to be Cu(II) by a mass/charge balance. Zn was oxidized to soluble Zn(II) leaving behind a trivial amount of solid Zn corrosion product on the surface. The kinetic analysis depicts a two-stage dissolution process of dezincification: a first stage of a rapid growth of the dezincified layer and a second stage where the growth of dezincified layer was much slower. The Cu2O layer grows continually during the exposure.
A vessel for the hardening of aerated concrete exploded after modification. The modification was implemented by welding additional barrels to extend the vessel. Two and a half years after the modification (about 500 load cycles) the vessel exploded showing a longitudinal crack starting in one of the new segments. After the modification the dimensions of the vessel were 2000 mm diameter and 31,000 mm length, operational pressures ranged from 0.3 to 16 bar. The material of both the old and new parts of the vessel and the properties of the welds showed no deviations from the specifications. The geometry of the longitudinal weld showed a roofing deviation from circularity. Fractographic investigations showed a crack of 1.6 m length following the longitudinal weld which developed over long time caused by corrosion fatigue. At the time of the explosion, the wall thickness was reduced from 13.5 mm down to about 2 mm leading to longitudinal cracking of the vessel throughout the new parts followed by a circular crack which resulted in a complete separation of the pressurized vessel. The crack development is shown by optical and SEM-micrographs.
Calculations of nominal stresses show that the geometrical deviations by linear misalignment and roofing cause a stress (tension load) increase of about 4.8 of the mean tensile load value. Adjacent to the weld this causes a local tension overload; in combination with operation conditions (overheated steam of 203 °C, cyclic operation) this causes corrosion fatigue due to cracking of corrosion layers. The crack formed by this mechanism caused a remaining wall thickness of less than 2 mm followed by a ductile residual fracture.