1

Repair Coatings for Thermally Sprayed Aluminium

Ole Øystein KNUDSEN1, Heidi ASKESTAD2, Rodgeir AANESEN3, Vincent

GREGOIRE4

1SINTEF, 7465 Trondheim, Norway, ole.knudsen@sintef.no

2NTNU, 7491 Trondheim, Norway, haskestad@gmail.com 3ConocoPhillips, 4056 Tananger, Norway, Aanesen, rodgeir.aanesen@conocophillips.com

4Statoil, 3936 Porsgrunn, Norway, vgr@statoil.com

Abstract: Thermally sprayed aluminium (TSA) is a highly resistant coating that is used in corrosive

environments for protection of steel. The experiences with TSA are very good, and long lifetimes can

be expected (decades). However, if the TSA is painted a special corrosion mechanism comes in action,

where the TSA will corrode rapidly and protection of the steel substrate is lost after a few years. A

crevice is formed between the steel and the paint coating, where TSA corrodes rapidly due to

acidification. The mechanism is parallel to traditional crevice corrosion, with a galvanic contribution

from the steel substrate. This introduces a problem for repair of damages in the TSA coating by

painting. The objective with the work has been to find coatings that can be used for repairing TSA

without triggering the TSA crevice corrosion mechanism.

Three hypotheses for how to avoid the crevice corrosion mechanism under the repair coating were

suggested:

• Paint coatings with a buffering capacity may neutralize the acidic environment inside the

crevice and slow down corrosion of the TSA

• Open coatings that will allow the acidic environment under the coating to diffuse out

• Cathodic protection by zinc, magnesium or aluminium rich paint

Based on the hypotheses, coatings were selected for experimental investigation. The results indicates

that both zinc, magnesium, magnesium oxide and aluminium particles in the repair coating are

beneficial. However, only the sacrificial coatings were able to protect areas where the TSA was

completely degraded and bare steel was exposed.

Keywords: Thermally sprayed aluminium; coating maintenance;

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Introduction

TSA has successfully been used for corrosion protection of steel for several decades [1-6].

Two modes of corrosion have been reported for TSA:

TSA exposed in a hot and wet environment under thermal insulation soaked with salt

water [7]

TSA covered by thick, protective coatings, exposed in corrosive environments [8-10]

The corrosion of TSA under wet thermal insulation is probably due to the very aggressive

conditions, with constant exposure to temperature, water and chlorides. The TSA is attacked

by general corrosion and is lost within a few years. As TSA is completely lost in areas, a

galvanic contribution from the bare steel will probably accelerate the corrosion further.

For painted TSA a corrosion mechanism similar to crevice corrosion has been proposed:

I. The corrosion rate is initially equal inside and outside of the crevice and the anodic

(corrosion of Al) and cathodic reaction (oxygen reduction) occurs on the entire metal

surface. Figure 1 shows the top-coated TSA with the electrochemical reactions

occurring at the metal surface and inside the crevice

Figure 1: Corrosion of TSA under the organic coating, forming a crevice.

II. The formation of OH- will stop when oxygen is depleted inside the crevice. The metal

dissolution inside the crevice is maintained by the oxygen reduction occurring outside

the crevice. The concentration of metal ions inside the crevice increases and the Cl--

ions (in chloride containing environments) migrates towards the crevice to maintain

charge neutrality. The migration leads to formation of aluminium chloride (AlCl3)

under the organic coating, which hydrolyse to form hydrochloride acid and aluminium

hydroxide (1):

AlCl3 + H2O = Al(OH)3 + 3 HCl (1)

III. The TSA activates when the pH decreases and the corrosion rate of TSA increases.

IV. The corrosion rate increases due to cathodic hydrogen evolution in the acidic

environment. The total corrosion reaction (2) of the TSA below the organic coating is:

2 Al + 6 HCl = 2 AlCl3 + 3 H2 (2)

Aluminium chloride is regenerated and the corrosion is self-sustained as long as water

is supplied.

This corrosion mechanism can easily be avoided by not applying thick, protective coatings on

TSA. However, when it comes to maintenance of TSA, painting is often the preferred method.

Application of new TSA is often out of the question due to limitations to hot work. Blasting

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off all remaining TSA and apply a new protective organic coating on the bare steel is usually

too expensive and time demanding to be an option. Hence, maintenance will be performed on

a mixed surface of remaining TSA and bare steel (where the TSA is completely corroded

away). It will then be difficult to avoid painting over the remaining TSA. Typically, an epoxy

mastic coating has been applied after surface cleaning, since epoxy mastics generally are

recommended for maintenance purposes due to their surface tolerance, i.e. can be applied on

surfaces that have suboptimal cleaning. This has resulted in short lifetime of the repair coating

due to the corrosion mechanism described above.

The objective with this work has been to find repair coatings for TSA that does not suffer

from the degradation mechanism described above, or at least have longer lifetime than

previously used maintenance systems. Three hypotheses were suggested for how to avoid the

degradation:

• Paint coatings with a buffering capacity may neutralize the acidic environment inside

the crevice and slow down corrosion of the TSA.

• Open coatings that will allow the acidic environment under the coating to diffuse out.

• Cathodic protection by zinc, magnesium or aluminium rich paint, to polarize the TSA

to a level where it is less susceptible to corrosion.

Ten different coatings or coating systems were selected. In addition, a reference system that is

known to cause the degradation problem was included. Another approach, with masking and

application of the repair coating edge-to-edge with the TSA was also tested.

Experimental

Materials

Steel panels of 5 mm thick hot rolled carbon steel was cut to 150 x 75 mm dimension, blast

cleaned and coated with 230 µm arc sprayed TSA (average). The TSA wire was AA 1050

alloy, i.e. 99% Al. The TSA was then blasted away in a circular area with diameter of

approximately 50 mm in the lower part of the sample, as shown in Figure 2. The repair

coatings were then applied according to the specification given in Table 1. The exception was

coating system no. 3, where the TSA was blasted away on half the sample, and the paint

system was applied only on the bare steel area, edge-to-edge with the TSA. In practice, there

was a gap of about 1-2 mm bare steel between the TSA and the paint system in order to avoid

any overlap. After the paint systems had cured for about 2 weeks, a 2 x 100 mm scribe

through the coatings down to the steel substrate was prepared as indicated in Figure 2, in both

bare steel and TSA covered areas.

All coatings were commercial products, except for no. 10, 11 and 12 that were prepared by

mixing aluminium powder, magnesium powder and magnesium oxide powder respectively

into the silicate binder from system no. 9 (the zinc silicate was supplied with the zinc powder

as a separate component).

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Table 1. Coatings tested for repairing damaged TSA

Repair coatings 1st coat 2nd coat 3rd coat

Sum

(wo/TSA)

Type DFT Type DFT Type DFT DFT

1 Reference Epoxy 25 Epoxy mastic 210 Polysiloxane 50 285

2

Zn epoxy

system Zn epoxy I 75 Epoxy mastic 210 Polysiloxane 50 335

3 Edge-to-edge Epoxy 25 Epoxy mastic 210 Polysiloxane 50 285

4 High T coating

Inorganic

copolymer 150

150

5 Zn epoxy I Zn epoxy I 90

90

6 Zn epoxy II Zn epoxy II 90

90

7 High Zn load High Zn load 90 High Zn load 90 180

8 Zinc phosphate

Zinc phosphate

epoxy 90

Zinc phosphate

epoxy 90 180

9 Zn silicate Zn silicate 90

90

10 Al silicate Al silicate 90

90

11 Mg silicate Mg silicate 90

90

12 MgO silicate MgO silicate 90 90

Figure 2. Test panels, where the TSA was blasted away in a 50 mm diameter area

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Methods

Three parallels of each coating system was tested according to ISO 20340 and evaluated with

respect to corrosion creep from the scribe in both the TSA area and the bare steel area, as well

as general degradation independent of the scribe. Corrosion creep reported is an average of six

measurements on each of the three parallels, i.e. 18 measurements. Results after 19 weeks

exposure are reported.

Barrier properties of the repair coatings applied on TSA were measured by electrochemical

impedance spectroscopy (EIS). Measurements were made with regular intervals over a period

of 130 days, during constant immersion in artificial seawater (ASTM D-1141). Impedance

spectra were obtained at -1200 mV vs Ag/AgCl, between 10 kHz and 1 mHz with a 10 mV

rms amplitude. Impedance at 1 mHz is reported.

Capacity for cathodic protection was measured for coatings applied on TSA. The samples

were immersed in artificial seawater. The protection capacity was measured by logging the

potential of the coated samples over a period of 70 days The potentials were logged daily

against a Ag/AgCl reference electrode.

Results

Corrosion creep results under paint on TSA and steel are given in Table 2. System no. 3, the

edge-to-edge system is evaluated with respect to corrosion creep under the paint from the

TSA-paint edge. Pictures of one parallel of each coating system are given in Figure 3, Figure

4 and Figure 5. The area where the TSA was blasted away is on the right side in the pictures.

The reference system gave a lot of corrosion on TSA but performed better on steel

Except for Zinc epoxy I, all the sacrificial coatings gave good performance on both

TSA and steel

The High T coating and the Al silicate were unable to protect bare steel. The High T

coating must be applied in two coats in order to protect steel. On TSA the performance

was much better than the reference system.

The MgO silicate and the zinc phosphate epoxy also performed better than the

reference system, both on steel and TSA

The edge-to-edge samples had no corrosion creep under the paint from the TSA-paint

edge.

Barrier properties of the coatings were measured by EIS (except coatings 1, 2, 3 and 7), and

the impedance at 1 mHz as function of time is given in Figure 6. The High T coating had a

high impedance initially, which gradually decreased during the exposure. The silicates and the

zinc rich primers showed the opposite development. The impedance was low from the start,

but increased during the exposure. Final impedances for all coatings was in the range 1000 to

10000 ohm.

Open circuit potentials (OCP) for coatings applied on TSA are given in Figure 7, measured

over a period of 70 days. The test probably measures a mixed potential of the TSA and

sacrificial pigments in the coating. E.g. the MgO pigments do not react electrochemically, so

for the MgO silicate the OCP is determined by the TSA.

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The white corrosion products on the surface of the repair coatings were scraped off from a

large area on each sample, and was analysed by ICP-MS. The compositions are given in

weight percent in Table 3. The corrosion products were a mix of aluminium oxide from the

TSA and oxide from the metal pigment in the repair coatings. Even though the analysis is

fairly accurate (about 2% relative standard deviations), the collection of the sample is not

necessarily representing the entire sample, so the percentages should only be taken as an

indication of the oxide composition.

Table 2. Corrosion creep from scribe

Repair coatings

Corrosion creep

under painted on

TSA

Corrosion creep

under paint on steel

Other degradation

[mm] [mm]

1 Reference 22 13

2 Zn epoxy system 5 1

3 Edge-to-edge N.A. 0

4 High T coating

3 To the edge Red rust penetrating the

coating on steel

5 Zn epoxy I

Blistering 0 Al2O3 and ZnO covering

the coating

6 Zn epoxy II

0 0 Al2O3 and ZnO partly

covering the coating

7 High Zn loading

0 0 Al2O3 and ZnO covering

the coating

8 Zinc phosphate 9 1

9 Zn silicate

0 0 Al2O3 and ZnO covering

the coating

10 Al silicate

4 To the edge Some Al2O3 on the surface

Red rust penetrating the

coating on steel

11 Mg silicate

2 2 Al2O3 and MgO partly

covering the coating

12 MgO silicate

0 3 Al2O3 and MgO partly

covering the coating

Table 3. ICP-MS analysis of surface oxide on the repair coatings

Coating system Surface oxide composition (%)

Al Zn Mg

5 Zn epoxy I 77 23

6 Zn epoxy II 67 33

7 High Zn loading 71 29

9 Zn silicate 39 61

11 Mg silicate 50 50

12 MgO silicate 66 34

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Figure 3. System 1 (reference); 2 (zinc epoxy); 3 (edge-to-edge); 4 (high T coating)

4

3

2

1

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Figure 4. System 5 (Zn epoxy I); 6 (Zn epoxy II); 7 (high Zn); 8 (zinc phosphate epoxy)

5

6

7

8

9

Figure 5. System 9 (Zn silicate); 10 (Al silicate); 11 (Mg silicate); 12 (MgO silicate)

9

10

11

12

10

Figure 6. Barrier properties of some of the coatings tested. Exposed area was 20 cm².

Figure 7. Open circuit potential of some of the repair coatings applied on TSA, exposed

in artificial seawater

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Discussion

As explained in the introduction, the coatings were selected based on three different

hypotheses for how a repair coating can avoid the crevice corrosion degradation mechanism:

Buffering reactions, open coatings that may allow an acidic environment to be washed out

and cathodic protection. All the selected coatings have at least one of the effects, except for

system 1, which is a reference system that is expected to give the problem and system 3 that

shall avoid the problem. Table 4 summarize which hypothesis the various systems are

assumed to satisfy.

Buffer coatings:

Zinc rich primers may act as a buffer, first by consuming acid by corrosion, and then

in a reaction with hydrochloric acid:

Zn + 2 HCl = ZnCl2 + H2

ZnO + 2HCl = ZnCl2 + H2O

Magnesium will react in a similar manner, but more powerful. Magnesium is thermo-

dynamically less stable than zinc, and magnesium oxide is a more alkaline oxide than

zinc oxide.

The zinc phosphate pigmented epoxy may act as a buffer and neutralize an acidic

environment under the coating.

MgO will have the same effect.

Cathodic protection:

The zinc rich and magnesium rich coatings may provide cathodic protection

Open coatings:

All the silicate based films have an open structure and may let an aggressive

environment under the coating leak out, or be washed away.

The high load zinc primer is also assumed to be quite open, like the silicates.

The zinc epoxies are more dense than zinc silicates, but may still be classified as open.

The aluminium rich silicate can probably only function as an open coating, since it is

reasonable to assume that the aluminium pigments are passive and provide no

sacrificial effect.

The High T coating was applied in only one coat at 150 µm, and the binder is probably

more open than most barrier coatings. Hence, it should also form a rather open film

that may let an aggressive environment leak out.

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Table 4. Possible mechanisms for the various repair coatings by which they may avoid

the crevice corrosion degradation mechanism

Repair coating Buffer Open coating Cathodic protection

1 Reference

2 Zn epoxy system X X

3 Edge-to-edge

4 High T coating X

5 Zn epoxy I X X X

6 Zn epoxy II X X X

7 High Zn loading X X X

8 Zinc phosphate epoxy X

9 Zn silicate X X X

10 Al silicate X

11 Mg silicate X X X

12 MgO silicate X X

The barrier property measurements in Figure 6 show that the High T coating initially acted as

a barrier, but that the resistance of the coating rapidly decayed, due to a certain degradation of

the coating. The TSA started to corroded locally, where the High T coating opened for ionic

transport. The other coatings had a low resistance against transport of ions at first, which then

gradually increased due to corrosion of the sacrificial pigments or the TSA. At the end of the

test all the coatings had a resistance between 1000 and 10 000 Ohm (resistivity 20 000 and

200 000 Ohm cm², taking the test area into account).

The open circuit potential measurements showed low potentials for all the samples in the end

of the test. The coatings with sacrificial pigments had low potentials from the start, while the

buffer coatings and the barrier coatings had higher potentials initially, the High T coating in

particular. The low OCP of the barrier and buffer coatings must be due to corrosion of the

TSA. Hence, the measurements cannot be taken as a proof of any cathodic protection effect

from the sacrificial pigments.

In the cyclic corrosion test, the reference coating was severely degraded, as expected. On

TSA, all the other repair coatings performed better, which must be taken as support to all the

three hypotheses that were proposed. Most of the coatings will work according to more than

one of the hypotheses, so it may be difficult to separate their effect. The zinc phosphate

pigmented epoxy can only work by a buffering effect, while the High T coating and the

aluminium silicate can only work by being open films. Hence, the fact that all these three

coatings gave better results than the reference system indicates that both mechanisms will

help decrease coating degradation on TSA.

Zinc epoxy I developed blisters over TSA during the test, while Zinc epoxy II did not. The

barrier properties and the capacity for cathodic polarization may explain the difference. Zinc

epoxy II had lower resistance (Figure 6) and gave lower OCP than Zinc epoxy I (Figure 7).

Hence, it had a more open structure and a higher capacity for cathodic polarization.

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Many of the coatings are typical primers, but have not been top-coated, except for Zinc epoxy

I, which was top-coated in Coating 2. Top-coating will make a dense barrier film, restricting

the function of the primer to buffering or cathodic protection. The cathodic protection may

also be limited, since only sacrificial pigments very close to the corroding TSA will be in

electrolytic contact with the TSA. In a sacrificial primer with no top-coat, pigments further

away will contribute by cathodic polarization. Hence, in a short-term perspective it will be

beneficial not to top-coat. However, the sacrificial pigments may react to fast if they are not

top-coated, losing their effect at an early stage. The oxides from the sacrificial primer will

also affect the visual appearance of the repair coating in a negative way. In the case of Zinc

epoxy I, top-coating improved the performance on TSA, since this particular product gave

severe blistering when applied alone on TSA. How the other primers will perform when top-

coated remains to be seen, but since they have performed better than Zinc epoxy I, even better

performance than Coating 2 may be expected.

Another important property of the repair coating is that it must be able to protect bare steel. In

order to protect steel we need either cathodic protection or a dense barrier coating. Hence, the

coating properties we need for protection of bare steel will partly be opposite to what we need

for a successful coating on TSA. The High T coating and the aluminium silicate failed on bare

steel. Both results were expected. The High T coating was applied thinner than recommended

by the supplier, while the Al silicate will have neither sacrificial effect nor sufficient barrier

properties.

Coating 3, which was applied edge-to-edge with the TSA, gave no corrosion creep from the

edge, in spite of a 1-2 mm gap of bare steel between the TSA and the repair coating. Hence,

this seems to be a good solution with respect to corrosion protection, if this can be done in the

field. The TSA have probably provided some cathodic protection of the gap. Degradation of

the organic coating may then rather be cathodic disbonding than anodic undermining. No

cathodic disbonding was found though.

Conclusions

Application of barrier coatings on TSA has previously caused massive corrosion problems,

assumed to follow a mechanism similar to crevice corrosion, with acidification of the

environment inside the crevice formed between the barrier coating and the TSA. The potential

repair coatings were selected to avoid the acidification. Three hypotheses for how to avoid the

acidification under the repair coating was proposed: (i) buffering pigments in the repair

coating, (ii) open coatings that may let the aggressive environment be washed out, and (iii)

cathodic protection. Ten coatings, satisfying one or more of the hypotheses, were selected and

evaluated as potential repair coatings for TSA. The repair coating must also be able to protect

bare steel. In addition, a barrier coating system was applied edge-to-edge with TSA.

All coatings performed better than the reference coating system that has been causing

problems previously.

It seems that all the three hypotheses proposed were verified by the testing

Most of the coatings tested can be categorized as primers with sacrificial pigments.

Only one of them were tested with a topcoat. The top-coating improved the

performance of this particular primer. Top-coating the other primers may give even

better results, but this remains to be tested.

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Applying the repair coating edge-to-edge with the TSA, without overlap, gives good

corrosion protection, even when there is a small gap of bare steel between. No

degradation was found under the repair coating.

The repair coating must also provide corrosion protection when applied on blasted

steel. Not all coatings satisfied this criterion.

Acknowledgements

Financial support from ConocoPhillips and Statoil is gratefully acknowledged.

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