Corrosion of Aluminium in BFS Composite Cement

1
Corrosion of Aluminium in BFS Composite Cement A.
Setiadi1, N.B. Milestone1, J. Hill2 and M
Hayes3 1Immobilisation Science Laboratory, Dept.
of Engineering Materials, University of
Sheffield, Mappin St, Sheffield S1 3JD 2Nirex
Ltd, Curie Avenue, Harwell, Oxfordshire, OX11
0RH 3Nexia Solutions, Sellafield, Seascale,
Cumbria CA20 1PG

• Introduction
• Composite cements (e.g. BFSOPC) are currently
  being used to encapsulate and immobilize
  radioactive intermediate level wastes (ILW) which
  can contain metallic aluminium.
• Aluminium is a very reactive metal, which when
  exposed to air, produces an oxide layer which
  generally provides a passive protective layer to
  further reaction.in the pH region 4.0 8.5 ( Fig
  1).

• SEM
• Fig. 6 shows details of the cross section of
  aluminium in cement From left to right,
  aluminium, corroded (affected) zone and the
  cement matrix.
• The hardened cement matrix (fig. 7a) looks very
  similar to the cement without any aluminium (fig.
  7b) indicating the Al corrosion has little effect
  on the bulk cement matrix.

Al(OH)3 S strätlingite, Ca2Al2SiO7.8H2O g
gehlenite, Ca2Al2SiO7 M- monosulfate,
4CaO.Al2O3.SO4.12H2O
Fig. 2 XRD of aluminium corrosion product of (a)
OPC and (b) 91 BFSOPC, ws 0.33, 20ºC, 90 days

• Fig. 8 shows the interface between the aluminium
  and the cement. In Fig 8a, the porous nature of
  the corrosion layer is seen along with some BFS
  grains.
• In Fig 8b, the metal interface is shown along
  where the metal surface appears to have undergone
  pitting corrosion and formation of crystals that
  resemble boehmite These crystals were not seen
  with the high purity Al sample confirming that
  the impurities in aluminium play an important
  role in rate and type of corrosion.
• This effect is also seen in the cross sectional
  samples where the corrosion ring around the
  Al-1050 is greater than the pure aluminium. (i.e.
  more corrosion with the Al-1050)

Fig. 3 DSC of aluminium corrosion product of (a)
OPC and (b) 91 BFSOPC, ws 0.33, 20ºC, 90 days
Fig. 1 Pourbaix diagram of reaction for
aluminium and iron

• In a cement grout, the alkaline environment (pH
  12 14), causes corrosion of aluminium.
• The passive layer dissolves to expose the bare
  metal.
• Al2O3 OH- H2O ? 2Al(OH)4.2H2O-
  (1)
• The bare metal then further corrodes
• 2Al 2OH- 10H2O ? 2Al(OH)4.2H2O- 3H2
  (2)
• Local loss of alkalinity results in deposition of
  AL(OH)3
• Thus corrosion results in volume expansion which
  may lead to cracks and hence failure of the waste
  form.

Fig. 4 Cross section samples of 91 BFSOPC with
(a) Al-1050 and (b) high purity Al.

• Microscopy
• On the Al-1050 sample, the presence of layers can
  be seen in the cross section of the corrosion
  product (Fig. 4).
• These layers are ca. 1.7 mm thick in total (Fig.
  5).
• For high purity aluminium, the corrosion layer is
  thinner than for Al-1050, at ca. 1.1 mm showing
  impurities enhance corrosion.
• .

Fig. 8 SEM (BEI) of 91 BFSOPC cement with
Al-1050 (a) porous region and (b) the aluminium
interface

• Experimental
• Cement blends 100OPC and 91 BFSOPC with w/s
  of 0.33 were used to study corrosion
• High purity (99.999) and grade 1050 (99.5) of
  aluminium examined
• Corrosion products studied by XRD, DSC, optical
  microscopy and SEM

• Mechanism
• A three stage mechanism is proposed for corrosion
  of aluminium in a hydrating cement matrix
• Firstly, the adherent protective layer on the
  aluminium surface dissolves producing aluminate
  ions (Eq. 1).
• Secondly, the soluble aluminate ions and silicate
  ions from the cement combine to produce
  strätlingite.
• Thirdly, the now bare aluminium corrodes
  producing aluminate ions and hydrogen gas (Eq.
  2). On depletion of OH-, the aluminate ions
  precipitate as aluminium hydroxide.

• Phase analyses
• Corrosion product extends for up to 2mm from
  aluminium rod.
• XRD shows that bayerite, Al(OH)3, and
  strätlingite, C2ASH8, are the main corrosion
  products. (Fig. 2)
• This is confirmed by DSC which shows peaks for
  Al(OH)3 at ca. 300ºC and strätlingite at 220ºC
  (Fig. 3)
• C2ASH8 is normally found in calcium aluminate
  cements with addition of silicates.
• Calcite, CaCO3, was found in most samples
  probably due to carbonation when handling the
  samples.
• Overall cement hydration has not been affected by
  corrosion. (at least up to one year)

• Conclusion
• Corrosion of aluminium in hydrating cements
  produces hydrogen gas and aluminium containing
  corrosion products, mainly aluminium hydroxide
  and strätlingite.
• The hydration of the cement is only affected
  close to the aluminium.
• The corrosion occurs in three stages dissolution
  of oxide layer, corrosion of bare metal and
  accumulation of corrosion products.

Fig. 5 Optical microscope of the interface
between Al-1050 and 91 BFSOPC, ws 0.33, 20ºC,
90 days.

• Acknowledgements
• Prof. J. H. Sharp (University of Sheffield) for
  helpful discussions.
• ISL and the University of Sheffield for funding
  the project

Fig. 6 SEM (BEI) of the interface between
Al-1050 and 91 BFSOPC, ws 0.33, 20ºC, 90 days.