We conclude our discussion of the different types of embrittlement mechanisms by talking about various steps that can be taken for the mitigation and/or protection of these effects.

To the extent possible, embrittlement can be “controlled” by a combination of good design, correct selection of resistant materials, environment management, maintenance and inspection. The source of stress can include one or more of the following: 

  • Operational conditions – applied (tensile) stresses
  • Thermally induced factors – temperature gradients, differential thermal forces (expansion and contraction)
  • Build-up of corrosion products – volumetric-dependent
  • Assembly issues – poor fit-up (tolerance problems), tightening, press and shrink fits, component interference, joining
  • Residual stresses from the manufacturing processes – joining (welding, brazing, soldering), forging or casting, surface treatment (plating, mechanical cleaning, etc.), heat treatment (quenching, phase changes), forming and shaping, machining, cutting and shearing

Steps to Mitigate Embrittlement Mechanisms

Various protection methods can be used to prevent a metallic system from corrosion, including:

  • Thermodynamic – selecting materials with a high (positive) value for the free-energy change for conversion of the free metal to a corrosive product
  • Kinetic – reducing the corrosion rate of the anodic or cathodic reaction by lowering current density
  • Barrier – isolation of the material from the corrosive medium by use of a coating, inhibitor, oxide layer, etc.
  • Structural design – minimizing exposure time to a corrosive environment
  • Environmental control – elimination of the principle constituent in a corrosive reaction from a (closed) environment
  • Metallurgical design – the proper alloy for the environment

Example: Hydrogen Embrittlement

To negate the effects of hydrogen embrittlement, it is necessary (singularly or in combination) to reduce hydrogen exposure, reduce hydrogen susceptibility, perform a hydrogen bake-out procedure and use relevant test mechanisms to ensure that hydrogen is not present in the material. Steps include:

  • Using lower-strength steels
  • Avoiding acid cleaning
  • Allowing time for hydrogen diffusion/removal after acid cleaning
  • Using low-hydrogen plating techniques
  • Reducing residual stress
  • Adding a bake-out step, typically 190-205°C (375-400°F) for 24 hours within 0.5-2 hours of plating

The following standards are commonly used to test for the presence of hydrogen:

  • ASTM F1113-87 (2011) – Standard Test Method for Electrochemical Measurement of Diffusible Hydrogen in Steels
  • ASTM F519-97e2 – Standard Test Method for Mechanical Hydrogen Embrittlement Evaluation of Plating Processes and Service Environments
  • ASTM F1624-00 – Standard Test Method for Measurement of Hydrogen Embrittlement Threshold in Steel by the Incremental Step Loading Technique

References

1. Herring, Daniel H., Hydrogen Embrittlement and Other Failure Mechanisms, The HERRING GROUP, Inc., 2011.