Staehle Consulting, Principal, Roger W. Staehle, Ph.D., NAE
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Approaching and resolving your questions
 

Backgrounds and subtleties to Staehle approach

Staehle has developed useful approaches that help his clients understand the peculiarities of corrosion.

  • All metals are chemicals; the surprise is not that they fail, the surprise is that they work.
  • No material has an innate strength; they only have the strength that they have in their present environment over the range of time of interest. This kind of strength is “situation-dependent strength.” Handbook values of strength are almost useless.
  • Design stresses are not usually relevant to failures in environments; the most relevant stresses are residual stresses, which are often three times the design stress.
  • Accumulations of deposits on surfaces are usually more damaging than the bulk environments.
  • The “mechanical-chemical” environment is much more damaging than either alone.  Such combinations include: slow creep in environments where the strain rate is in the range of 10-6 to 10-9/sec; combined wear plus environments; fretting plus environments; cyclic stressing at low cyclic frequency in environments.
  • Superheat and/or evaporation produce large concentrations of usually corrosive species such as chloride.  0.01ppm Cl can become 100% in a short time.
  • The initiation of SCC on surfaces are dominated by initial surface abuse.
  • The most dominant concept in corrosion is defined by the Pourbaix diagrams, e.g. the E-pH diagram, i.e. the thermodynamic stability at a given pH and electrochemical potential.
  • Increasing strength by cold work decreases the absolute threshold for SCC significantly.  Thus cold work is not an advantage for strength.

Cold worked surface diagram

  • Steady state operation is only one of the environments that needs to be considered in planning for corrosion; need also to consider startup, shutdown, and previous manufacturing.
  • KIc is not a criterion for anything except in vacuum; only KIscc is dominating; even KIscc is time-dependent.
  • Strength properties in environments are distributed over broad ranges of stress and temperature.  There is no single value; there are only distributions.
  • Corrosion rates depend less on galvanic series than on the catalytic properties, i.e. the exchange current density.

This array of trends often makes corrosion counter-intuitive.

SCC at weld matrix interface in stainless steel pipe in pure water

Typical questions from clients

  • How could this failure occur in pure water?
  • The chloride was less than 0.1ppm; how could this failure occur happen?
  • Accelerated testing showed that this failure should not have occurred; why did it?
  • How can pure copper fail by SCC without ammonia?
  • How can C-22 alloy fail so easily?
  • How can stainless steel fail—after all, it’s stainless?

Clients first define the problem, the phenomenology

When the failure is first brought to Staehle, he asks clients to define their problem, where they can, in detail: 

  • Evidence of failure.
  • When installed?
  • When was failure discovered?
  • Supplier?
  • Environment in which failure occurred?
  • Nominal chemical compositions of material?
  • How many failures of similar units?
  • Nature of guarantee or expected life in what environments?
  • Specifications?
  • Location of failure, e.g. near welds, bends, bottom, top etc.?
  • Drawings?
Progressive Corrosion

Client describes site of failure

Staehle also asks the client about:

  • Physical surroundings, local environments.
  • Chemistry of local environments, present and over time.
  • External forces.
  • Use over time: steady state, cyclic behavior, upsets.
Concentrated environment at intersection between tube and tube supporrts in the steam generator of a pressurized water reactor.

Special examinations as needed

Special equipment used at laboratories where high-quality equipment is located, significantly reducing overhead cost to Staehle Consulting.  Staehle supervises all off-site examinations and charges-through subcontracting work at cost without markup.

  • Examples of equipment used:
  • Physical features of failure with optical microscope.
  • Detailed features with scanning electron microscope (SEM).
  • Chemistry of materials.
  • Local chemistry (EDS).
  • Auger electron spectroscopy (AES).
  • Scanning ion mass spectroscopy (SIMS).
  • Chemistry of environment, water chemistry.
  • Residual stresses with X-ray.
  • Analytical transmission electron microscope (ATEM).
  • Electrochemical measurements, polarization.

Collaboration

In complex problems, other experts, who are well known by Staehle to be experts in their respective fields, may be invited to collaborate.  All costs for collaborations will be previously discussed and approved by client.