VIEWS: 242 PAGES: 33

									      CHAPTER 3
          3-43 Mechanism

• more effort and funds have been

 expended on stress corrosion than an

 all other forms of corrosion combined
   Two basic “models” for a general
mechanism are
    (1) the dissolution model wherein
anodic dissolution (Fig 3-79) occurs at
the crack tip because strain ruptures the
passive film at the tip.
    (2) the mechanical model, wherein
specific species adsorb and interact
with strained metal bonds and reduce
bond strength.
• The first seems more universal than
 the second.

• For   most   engineering        work    past
 experience is the best guide, with
 reliable and valid testing second

• In all cases chemistry, metallurgy, and
 mechanics     (stress   field)    must    be
 3-44 Multi environment Charts
• Several     charts     (tables)      showing
 cracking tendencies for metal and
 alloy      systems     in   a      variety   of
 environments including liquid metals.

• These     tables     are   from     Materials
 Technology Institute of the Chemical
 Process Industries, Inc.
Table 3-13 shows the situation for carbon steels.
• Low-alloy   steels   with   very   high

 strengths (i.e., AISI 4340) are much

 more susceptible to stress corrosion

 than weaker steels.

• In general, susceptibility to cracking

 increases with strength level.
Figure 3-72 Schematic diagram showing potential ranges over
    which SCC of carbon steels occurs in various solutions.
• This means that changes in solution

 composition    or   temperature   could

 shift the potential of the system into

 or out of the danger zone. Cathodic

 protection and/or inhibitor additions

 can inhibit cracking.
Table 3-14 Environments vs. Various wrought stainless steels
Table 3-15 Environments vs. Copper alloys
Table 3-15 Environments vs. Aluminium alloys
• Table 3-17, Zirconium and its alloys are

 resistant to stress corrosion in pure

 water, moist, air, steam, and many

 solutions of sulfates and nitrates.
• It could crack in FeCl3 and CuCl2

 solutions, halogens in water, halogen

 vapors, and organic liquids such as

 carbon tetrachloride, and fused salts

 at high temperatures
• Columbium (niobium) and tantalum

 are   not   subject   to   usual   stress

 corrosion. They can be embrittled by

 hydrogen.    This     embrittlement    of

 tantalum in hot acids can be inhibited

 by contact with platinum.
• Magnesium alloys are being used in

 many cases where light weight is an

 important    factor,   contrary   to    the

 general     impression    of   the     poor

 corrosion resistance of magnesium.
• Alloys containing manganese have

 good resistance, but those with high

 aluminum or zinc content are quite

 susceptible to stress corrosion.
3.45 Classification of Mechanisms.
• The complexity of the interactions

 between various environments, nature

 of the alloy, metallurgical structure,

 etc., indicates the impossibility of one

 unified mechanism for stress corrosion

 of all metal-environment systems.
• M.A. streicher classified some SCC
  mechanisms that may be operative in
  different systems as follows :

    a. Dislocation coplanarity. Resistance

to cracking corresponds to the dislocation


   - stainless steels = planar arrays

   -resistance alloys = cellular or tangled
  b. Stress-aging and microsegregation

    -   stress   aging    of   austenitic

stainless steels  jerky plastic flow.

This phenomenon is associated with

microsegregation of solute atoms to

dynamic defects in the crystal structure.
    c.    Adsorption.     Surface      active

species    adsorb   and     interact    with

strained bonds at the crack tip, causing

reduction in bond strength and leading

to cracking propagation.
  2. Dissolution Mechanisms.
    a. Stress-accelerated dissolution.

Crack propagates by localized anodic

dissolution. Principal role of plastic

deformation   is   to   accelerate   the

dissolution process.
   2. Dissolution Mechanisms.
    b. Film-Formation at Cracking Wall.

As the crack progresses, the film on the

crack walls is repaired and serves as a

cathodic site.
   2. Dissolution Mechanisms.
      c. Noble element enrichment. The
copmosition of the slip step has a lower
nickel concentration than that to the
enriched surface; and the slip step
dissolved until the nickel is enriched to
the     same   composition     as    the
preexisting surface.
   2. Dissolution Mechanisms.
    d. Film rupture. Stress-corrosion

cracking   proceeds      by   successively

breaking a passive film. At the point of

rupture,   dissolution    proceeds   until

repassivation occurs.
   2. Dissolution Mechanisms.
    e.     Chloride   ion   migration.   The

chloride     ion   migrates   through    the

cracked film toward the region of

highest stress. The chloride ion then

acts to break down the film, Thereby

allowing metal dissolution.
    3. Hydrogen Mechanisms
    a.   Hydride   formation.    Hydrogen

enters type 304 stainless steel to from

martensite,   which   will   diffuse   into

stringers normal to the direction of

stress and the cause cracking.
    3. Hydrogen Mechanisms
    b. Hydrogen embrittlement. Hydrogen

accumulates within the metal at the crack

tip, leading to localized weakening, either by

void formation or lowering the cohesive

strength. Cracks propagate by mechanical

fracture of the weakened region.
   4. Mechanical Mechanisms
    a. Tunnel Pitting and Tearing. Crack

propagates by formation of deep pits or

tunnels via dissolution followed by

lingking of these pits or tunnels by

ductile rupture.
   4. Mechanical Mechanisms

    b. Corrosion Product Wedging. The

corrosion products build up in existing

cracks and then exert a wedging action.
   3.46 Methods of Prevention
      6. Coatings - Keeping the environment
away from the metal.
      7. Shot-peening (also known as shot-
blasting) produces residual compressive
stresses in the surface of the metal. Woelful
and      Muhall    show     very    substantial
improvement       in   resistance   to   stress
corrosion as a result of peening with glass

To top