7/28/2019 Bautista Et Al._sealing Mechanism of Anodized Aluminum

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Sealing Mechanism of Anodized Aluminum in

Triethanolamine Solutions

 A. Bautista* , J.A. González

**and V. López

**

 *Universidad Carlos III de Madrid

Avda. Universidad nº 30, 28911-Leganés, Madrid (Spain)**

Centro Nacional de Investigaciones Metalúrgicas

(CSIC)

Avda. Gregorio del Amo nº 8, 28040-Madrid (Spain)

The ability of triethanolamine (TEA) to

accelerate the sealing of anodic coatings on aluminum at

boiling and lower temperatures is already known (1,2).

The use of Electrochemical Impedance

Spectroscopy (EIS), as well as traditional standard control

quality tests (dye spot, phosphochromic acid dissolution

and admittance at 1 kHz tests) (3) and the study of mass

changes during the process, have made possible to

analyze the sealing mechanism in TEA solutions, and itscomparison with the proposed mechanism for traditional

sealing in boiling, de-ionized water (4).

Using the same techniques, the study of the

sealing in solutions of other different organic compounds

with amine and/or alcohol groups in their structure has

been carried out to find out which functional group gives

TEA its catalytic effect. The obtained results show that,

although both kinds of functional groups accelerate the

hydration of the alumina, the presence of alcohol and

amine groups in the same molecule has a synergic effect

(fig. 1)

Sealings obtained in TEA at different pHs have

also been compared with sealings obtained in other

amines and alcohols solutions at the same pHs, to

distinguish the effect of the pH of the TEA solutions (9.7)

from the effect of the functional groups. The slight

basicity of the TEA solutions have demonstrated to favor

the sealing, but the catalytic effect of the TEA is due not

only to its pH but also to its specific chemical structure.

Sealing Mechanism

During sealing in TEA solutions, 4 steps can be

distinguished (fig. 2):

 Fig. 1- Porous layer resistance after 24 h sealing at 50

ºC in 2 ml/l solutions of different organic compounds.

1st

step: Pore mouth plugging due to hydrated compound 

 precipitation on the surface and partial pore filling with

water. TEA considerably reduces the minimum sealing

times to surpass the standard tests that exclusively depend

on the condition of the surface of the coating (dye spot

and acid dissolution tests). EIS also shows a faster

increase in the resistance corresponding to the more

external region of the coating during sealings in TEA than

during sealing in de-ionized water, while the

electrochemical parameters of the more internal regions of 

the coating evolve in a more similar way. These facts

suggest that TEA favors the formation of the surface layer

that blocks the pore mouth before the end of the water

absorption step.  

2nd

step: Saturation of the pore volume with water 

absorbed through the surface layer of hydrated 

compounds. After pore mouth plugging, water absorption

-and the consequent reaction with pores walls- continue

until the mass gain reaches a saturation value. This value

corresponds with a water volume higher than the pores of the anodic coating could initially contain. The pH of TEA

solutions favors partial dissolution of the alumina and

make the absorbed water amount higher than during

traditional sealing.

3rd

step: Precipitation of hydrated compounds inside the

 pores. During this step, it is more difficult to detect the

effect of TEA, and the hydrated reactions must occur at

quite similar rate in TEA and de-ionized water.

4th step: Conversion of hydrates into more stable forms.

Aging of anodic layers lasts for years, erasing initial

differences between sealings obtained in TEA andsealings obtained in de-ionized water.

References:1.  J.A. González, E. Otero, A. Bautista and V. López,

Plat. Surf. Finish., 84 (nº 7), 59 (1997).

2.  A. Bautista, E. Otero, V. López and J.A. González,

Plat. Surf. Finish., 85 (nº 5), 110 (1998).

3.  ISO Standards 2143 and 2931.

4.  J.A. González, V. López, E. Otero and A. Bautista,

 J. Electrochem. Soc., (accepted for publication).

Fig. 2 – Proposed sealing mechanism for porous alumina

layers in TEA solutions

0

10

20

30

40

50

60

70

80

90

10 0

   R  p   (   k   O   h  m  ·  c  m   2   )

  G   l  y  c  e

  r   i  n

   E  t   h  y   l  e  n

  g    l   i  c

  o   l

   E  t   h  a

  n  o   l  a

  m   i  n

  e   T   E

  A 

   T  r   i  e

  t   h  y   l  e  n  a

  m   i  n

  e

   E  t   h  y

   l  e  n  d   i  a

  m   i  n

  e

   N  o  n

  e

pH=6-7

Without pH

adjustment