Untersuchungen zum Einfluss von Inhibitoren auf die Lochkorrosion Typ I in Trinkwasser-Installationen aus Kupfer

In recent years damages of pitting corrosion in domestic drinking-water installations of copper could be observed. The pitting likelihood is determind by the material, the processing, the operation and the corrosivity of the drinking-water. But the corrosion damage is generally influenced by an unfavourable combination of all these factors. Until today, not all limiting conditions of pitting corrosion of copper in drinking-water are yet known, e.g. initiation, stabilization and repassivation of pitting corrosion, particularly with regard to activation of repassivated pitting at aged copper tubes when the drinking-water quality is changed significantly. Additionally, there are no reliable informations about the possibility to solve pitting corrosion problems by water treatment, i.e. dosage of drinking-water corrosion inhibitors. The extraction of general findings is often very difficult because in practice, there are often only isolated cases of pitting corrosion damages. Added to that, the history of these damages could be rarely reconstructed. For methodical investigations, not just a sufficient number of defects are needed but the start-criteria must be directly comparable. Copper pitting corrosion is determined by a great number of different factors, so empirical investigations show that the test conditions must be hard enough to initiate and stabilize pitting corrosion under defined conditions. As a result, corrosion stimulating factors (by analysing literature and completing by own pre-investigations) were combined and taken into account; but the investigations were in accordance with the real conditions of copper tubes in drinking-water installations. Until 2003, in Germany damages only occur in drinking-water installations with hard copper tubes (R 290, EN 1057) which were connected by hard soldering (general joining techniques until 1996 in Germany) or which were heated to bend (without the use of fittings). Under corrosion promoting conditions which were introduced by local heat treatment of copper tubes simulating hard soldering and 20 weeks horizontal half-filled pre-exposure of the tubes, the copper corrosion behaviour was studied in field corrosion tests for a period of two years. 256 heat treated copper tubes were exposed to a hard ground water containing high contents of natural salts and the same water after chemical softening (rapid decarbonisation). The test conditions were similar to those in a domestic installation with respect to stagnant and flow conditions. After different exposition times copper tubes were removed and examined with respect to morphological examinations of localized attack, measurements of surface roughness, measurements of all pitting attacks in a defined area, maximum corrosion depth measurements and statistical characterization. The group of data presenting maximum corrosion depth were analyzed by extreme value statistical analysis. The combination of the measurement of all pitting attacks and the extreme value statistical analysis showed that a stabilized increase of pitting only occur by pitting depths significantly higher than 100 µm. Localized attacks were observed only in characteristic areas of the heat treated zone. The results illustrate, that there are eminent differences in corrosion behaviour. In the hard ground water, shallow pit formation could be observed, in the softened water pitting corrosion. The shallow pit formation corresponded to the water line which was induced by partly filled exposure; so the operating conditions – i.e. the pre-exposure of the copper tubes – played a dominant role. In the softened water, pits were randomly distributed in the area of high localized pitting attack but not preferently in surfaces which had been in water contact during half-filled exposure. With rising test period, pitting corrosion was stabilized by contact with the softened water. In the hard water the pits repassivated in different depths (not exceed 100 µm) and active pitting corrosion stopped. In a second step, the influence of drinking-water suitable corrosion inhibitors (phosphate and/or silicate) on pitting corrosion was studied. The tests have shown that pure phosphate (4.5 mg/l total PO4, (75% ortho- and 25% polyphosphate)) supports the repassivating tendency in both waters. The pure silicate inhibitor (12 mg/l SiO2) promote pitting in the hard water, while in the softened water both surface conditions - repassivating and active pitting - occurred. The mixed inhibitor (2 mg/l PO4 / 3 mg/l SiO2) didn’t have any influence in softened water combined to the untreated water; in hard water, also repassivation of pits were observed, but in contrast to the development by dosing pure phosphate the process of repassivation started later.

Grantor dc:publisher

Publikationsserver der RWTH Aachen University

Year dc:date

2009

Author dc:creator

• Becker, Angelika

Contributors dc:contributor

• Schmitt, Günter

• info:eu-repo/classification/ddc/620
• Korrosion
• Kupfer
• Korrosionsinhibitor
• Trinkwasser
• Extremwertstatistik
• Ingenieurwissenschaften
• corrosion
• copper
• corrosion inhibitor
• drinking-water
• extreme value statistics

Statement dc:rights

• info:eu-repo/semantics/openAccess

Language dc:language

ger

Identifier

https://publications.rwth-aachen.de/search?p=id:%22RWTH-CONV-113795%22

OAI record oai:identifier

https://publications.rwth-aachen.de/oai2d?verb=GetRecord&metadataPrefix=oai_dc&identifier=oai%3Apublications.rwth-aachen.de%3A51509