Wasserstoffinduzierte Spannungsrißkorrosion an Blechen aus hochfesten Dual- und Komplexphasenstählen

Translated title of the contribution: Hydrogen Induced Stress Corrosion Cracking of Ultra-High Strength Steel Sheet

Roland Scharf

Research output: ThesisDoctoral Thesis

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Abstract

Zinc coated steel sheets for corrosive protection are commonly used in the automotive industry. Increasing need of weight reduction pushes the substitution of components made of conventional carbon steels by Advanced High Strength Steel grades. These are multiphase grades with an ultimate tensile stress level above 1200 MPa. Corrosion protection layers as well as the microstructure affect the aim of this work to predict the chance of hydrogen induced stress corrosion cracking. The preferred method of choice has been constant load tests, using notched tensile specimens and adjusting in-situ corrosive hydrogen charging. Crack initiation indicates the uptake of a critical hydrogen concentration. Parameters influencing the hydrogen susceptibility are fracture toughness, corrosion and electrolyte conditions, specimen geometry and implemented material defects at the crack initiating region. The fracture toughness of complex and dual phase steel grades was quantitatively characterized by mechanical tests and compared with the results from hydrogen sensitivity testing (constant load test). Flat steel sheet specimen preparation methods showed a strong effect on the hydrogen sensitivity of the specimens. Introduced microcracks, work hardening or phase transformation by laser cutting, punching or milling are discussed in respect to crack initiation at the cut edge by hydrogen. The hydrogen absorption rate as function of electrolyte volume, electrolyte pH and electrolyte convection of bare and zinc coated specimens was characterized. By use of zinc coated specimens only the zinc dissolution rate determines the hydrogen absorption rate on the bare steel surface of the cut edge (Zn --> Zn2+ + 2e- und 2H+ + 2e- --> H2 or 2 Hab). This adsorption rate is influenced by the initial electrolyte pH at test start, convection and the electrolyte volume. Under most test conditions a low pH, high convection and a huge electrolyte volume lowered the resistence tohydrogen embrittlement resistance. Exceptions are discussed. Passivation due to quick electrolyte change in near surface areas as a result of the corrosion reactions or passivation of the steel surface due to high pH-values and strong convection can cause such exceptions.
Translated title of the contributionHydrogen Induced Stress Corrosion Cracking of Ultra-High Strength Steel Sheet
Original languageGerman
QualificationDr.mont.
Awarding Institution
  • Montanuniversität
Supervisors/Advisors
  • Schenk, Johannes, Assessor B (internal)
  • Mori, Gregor Karl, Assessor A (internal)
Publication statusPublished - 2018

Bibliographical note

embargoed until 03-04-2023

Keywords

  • galvanic corrosion
  • hydrogen embrittlement
  • steel coatings
  • ultra-high strength steel sheet
  • automotive application

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