Corrosion protection for actuators of industrial equipment

Dietmar Isele, Andreas Baumgart, AUMA Riester GmbH & Co. KG, Müllheim, Jochen Keller, Emil Frei GmbH & Co. KG, Bräunlingen, Germany

1. Introduction
A practical report on the changeover from paint to powder coatings.
Discussions include coating technology, component design and plant operation as factors affecting the finished coating. The objective was compliance with corrosion protection requirements of class C5-M according to DIN EN ISO 12944-6.

2. AUMA customers and their benefits
Actuators are key components in systems controlling the flow of material and thus for the safety and economic viability of entire industrial installations. The various energy and material cycles in every plant must be individually designed from scratch for each industrial process.

Most of AUMA's customers are operators of industrial installations in which valves have to be driven, regulated or controlled. Actuators are also required by valve manufacturers and companies planning industrial installations. Wherever any kind of valve - be it a ball, gate or butterfly valve - is needed to open and close industrial pipelines, AUMA can supply the appropriate actuator with the required torque and the associated control unit.


Fig. 1: AUMA customers:
Abbildung_1_1Abbildung_1_2

2.1 Application fields of AUMA products
       

Water management
Wasserwirtschaft
Energy sector
Energiewirtschaft
Industrial facilities
Industrieanlagen
Pipelines
Pipelines
Petrochemical industrie
Petrochemische-Industrie
Shipbuilding
Schiffbau

Fig. 2: Application fields


3. Coating and coating technology
3.1 Two-layer powder coating system

In most cases, even very demanding requirements for a coating can be fulfilled with a single layer powder coating. However, in case of extreme requirements, particularly with regard to corrosion protection, a two-layer coating system was sought.

 

Powder coating

Basis

Curing

Baking conditions

Application

Layer thickness

Base powder coating
PE1204A

Epoxidharz/ Dicyandiamid

Polyaddition

10 min/160 °C

Corona Tribo

60-80 µm

Top powder coating PU4003M

epoxy resin/ dicyandiamide

polyaddition

10 min/160 °C

Corona

60-80 µm

Fig. 3: Structure of the vertical two-layer powder coating system

3.2 Base powder coating PE1204A
This consists of a specially modified epoxy resin with a high cross-linking density and very good adherence to various metal substrates. Energy-efficient powder coatings produce the required properties at cross-linking temperatures of 160 °C and above.

 

 

Colour

Gloss level

Density

Cross-cut test

Erichsen test

Falling ball impact test

 

EN ISO 2813

 

DIN EN ISO 2409

DIN EN ISO 1520

DIN EN ISO 6272

RAL 7035

glossy

1,579 g/cm³

Gt 0

(steel plate)

5 mm

40 kg cm (denting)

 Fig. 4: Properties of PE1204ARA735

Why does it not contain zinc powder?
■ Zinc powder does not exert its electrochemical, cathodic corrosion protection effect.

- Test results from IKS Dresden (FARBE+LACK 4/2004)
- Test results from FreiLacke (EFD-Info 119 / 2003)

■ Poorer handling properties
■ Surface imperfections in the top layer (due to coarse particles in the zinc powder)
■ High wear of the coating application equipment
■ Higher density - poorer economic viability


3.3 Top powder coating PU4003M
The top layer is made of a polyurethane powder coating based on a polyester resin, cross-linked with a physiologically harmless, non-cleavable isocyanate. Both layers must be fully cured at 200 °C to achieve the required properties. The two powder coatings are optimally matched with respect to the relevant properties. In particular, one of the key properties is the excellent adhesion between the base and top powder layers.

 

Colour

Gloss level

Density

Cross-cut test

Erichsen test

Falling ball impact test

 

EN ISO 2813

 

DIN EN ISO 2409

DIN EN ISO 1520

DIN EN ISO 6272

ca. RAL 9007

semi-glossy

1,631 g/cm³

Gt 0 (steel plate)

5 mm

40 kg cm (denting)

Fig. 5: Properties of PU4003M

Why polyurethane?
■ Chemical resistance
■ Resistance to UV and weathering
■ Operating temperatures of 80 to 120 °C
■ Glass transition temperature
■ Decontaminable

Thus this high-quality coating system has an excellent overall performance because
■ it has very good adhesion to the substrate,
■ there is excellent adhesion between the base layer and the top layer,
■ the top layer has good resistance to UV and weathering,
■ it acts as a first-rate barrier against permeation of harmful substances.

3.4 Standards
In the industrial coating industry, special powder coating systems are competing with conventional multilayer liquid coatings, especially in the corrosion protection sector.
Owing to the statutory requirement to reduce VOC emissions, the trend towards increasing use of environmentally friendly powder coatings will continue in future.
The most important standards for off-shore corrosion protection in Europe are DIN EN ISO 12944-6 (Corrosion protection of steel structures by protective paint systems), ISO 20340 (Paints and varnishes - Performance for protective paint systems for offshore and related structures) and NORSOK M 501 (Surface preparation and protective coating).

To date, DIN EN ISO 12944-6 regulates only systems with liquid coating materials. Although the inclusion of powder coating systems into the set of standards is urgently required, it is still not covered.
The use of special two-layer powder systems is interesting for all coating shops wishing to guarantee a high standard of corrosion protection on marketed goods.
Likewise, they are also of interest to users who would like to switch over to powder coatings in order to comply with the statutory VOC stipulations on reducing the amount of solvent emissions from liquid coatings. The powder coating method has the lowest emission of < 0.1 % during the cross-linking process.
The quality classification based on DIN EN ISO 12944-6 is often used to verify compliance with the currently valid requirements for steel structures and to allow comparisons with commercially available liquid coating systems.

A key element of DIN EN ISO 12944 is the classification into corrosivity categories in conjunction with the expected durability of the coating.

Corrosivity category

Load

Environment Indoor

Environment Outdoor

C 1

very low

heated buildings offices, schools

--

C 2

low

unheated buildings, condensation,

sports halls, schools

atmosphere with low pollution rural areas

C 3

medium

production facilities with high humidity and some pollution

laundries

urban and industrial atmospheres, polluted with SO2

coastal regions with low salinity

C 4

high

chemical plants, swimming pools, boathouses, seawater

industrial areas & coastal regions with moderate salinity

C 5 – I

very high

areas with almost constant condensation + heavy pollution

industrial areas with high humidity + aggressive atmosphere

C 5 – M

very high

areas with almost constant condensation + heavy pollution

coastal and offshore regions with high salt load

Fig. 6: Corrosion classes according to DIN EN ISO 12944
 

 

Corrosivity category

Durability of the coating

Salt spray test ISO 7253

Tropical test ISO 6270

Resistance to chemicals ISO 2812-1

 

(expected durability)

[h]

[h]

[h]

C 2

short 2-5 years
medium 5-15 years
long > 15 years

--

48

48

120

--

C 3

K

M

L

120

240

480

48

120

240

--

C 4

K

M

L

240

480

720

120

240

480

--

C 5 – I

K

M

L

480

720

1440

240

480

720

168

168

168

C 5 – M

K

M

L

480

720

1440

240

480

720

--

Fig. 7: Durability according to DIN EN ISO 12944


The international standard ISO 20340 is essentially based on DIN EN ISO 12944. The corresponding qualification requires compliance with corrosivity category C 5 - M with a durability of „long". In contrast to the coating structures defined/limited in DIN EN ISO 12944-5, this is left intentionally open in ISO 20340.


3.5 Test results

In accordance with DIN EN ISO 12944: compliance with C5-I long and C5-M long.

3.5.1 Salt spray test according to DIN EN ISO 9227 NSS

Evaluation after 1512 h

Grey cast iron

Die cast steel

Degree of blistering

DIN EN ISO 4628-2

0-0 (S0)

0-0 (S0)

Delamination at cut edge

DIN EN ISO 4628-8

< 0,5 mm

0 mm

Degree of surface rusting

DIN EN ISO 4628-3

Ri 0

Ri 0

Fig. 8: Salt spray test – evaluation after 1512 h, substrate grey cast iron and die cast steel
 

 

Abbildung_09_-_1512hFig. 9: Salt spray test grey cast iron, zinc-phosphated, 1512 h

 

 

Abbildung_10_-_1512h Fig. 10: Salt spray test die cast steel, zinc-phosphated, 1512 h

 

 

Evaluation after 2016 h

Grey cast iron

Die cast steel

Degree of blistering

DIN EN ISO 4628-2

0-0 (S0)

0-0 (S0)

Delamination at cut edge

DIN EN ISO 4628-8

0,5 mm

0 mm

Degree of surface rusting

DIN EN ISO 4628-3

Ri 0

Ri 0

Fig. 11: Salt spray test – evaluation after 2016 h, substrate grey cast iron and die cast steel


 

Abbildung_12_-_2016h
Fig. 12: Salt spray test grey cast iron, zinc-phosphated, 2016 h

 

Abbildung_13_-_2016h
Fig. 13: Salt spray test die cast steel, zinc-phosphated, 2016 h

 

 

Evaluation after 2520 h

Grey cast iron

Die cast steel

Degree of blistering

DIN EN ISO 4628-2

0-0 (S0)

0-0 (S0)

Delamination at cut edge

DIN EN ISO 4628-8

0,5 mm

0 mm

Degree of surface rusting

DIN EN ISO 4628-3

Ri 0

Ri 0

Fig. 14: Salt spray test – evaluation after 2520 h, substrate grey cast iron and die cast steel
 

 

Abbildung_15_-_2520h Fig. 15: Salt spray test grey cast iron, zinc-phosphated, 2520 h

 

Abbildung_16_-_2520hFig. 16: Salt spray test die cast steel, zinc-phosphated, 2520 h


3.5.2 Tropical test in accordance with ISO 6270
 

Evaluation after 1008 h

Grey cast iron

Die cast steel

Degree of blistering

DIN EN ISO 4628-2

0-0 (S0)

0-0 (S0)

Delamination at cut edge

DIN EN ISO 4628-8

0,0 mm

0 mm

Degree of surface rusting

DIN EN ISO 4628-3

Ri 0

Ri 0

Fig. 17: Tropical test - evaluation after 1008 h, substrate grey cast iron and die cast steel

3.5.3 Chemical resistance according to ISO 2812-1

 

Evaluation after 480 h

Grey cast iron

Die cast steel

Degree of blistering

DIN EN ISO 4628-2

0-0 (S0)

0-0 (S0)

Delamination at cut edge

DIN EN ISO 4628-8

0,0 mm

0 mm

Degree of surface rusting

DIN EN ISO 4628-3

Ri 0

Ri 0

Fig. 18: Kesternich test - evaluation after 480 h, substrate grey cast iron and die cast steel

All tests of the two-layer system that are relevant to compliance with the standards were carried out on a variety of substrates. The results from the salt spray test showed that grey cast iron, which is much more
susceptible to corrosion, exhibited a maximum corrosion creep of 0.5 mm after a test duration of 2520 h, which significantly exceeds the requirements of the standards. The results from the tropical test and the test of chemical resistance showed no changes with respect to adhesion, discolouration, blistering or corrosion.

 

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Degree
of blistering

Degree
of
rusting

Degree
of
cracking

Degree
of
flaking

Corrosion at the scribe

Pull-off test

         

0.05 mm scribe

2 mm scribe

unloaded

loaded

Requirement

0-0 (S0)

Ri 0

Class 0

Class 0

< 1 mm

< 3 mm

---

At least 50 % of the initial value

Grey cast iron 2

0-0 (S0)

Ri 0

Class 0

Class 0

1 mm

1 mm

>7 N/mm2

>7 N/mm2

Grey cast iron 3

0-0 (S0)

Ri 0

Class 0

Class 0

1 mm

1 mm

>7 N/mm2

>7 N/mm2

Grey cast iron 4

0-0 (S0)

Ri 0

Class 0

Class 0

---

---

>7 N/mm2

>7 N/mm2

                 

Die cast steel 1

0-0 (S0)

Ri 0

Class 0

Class 0

None

None

>7 N/mm2

>7 N/mm2

Die cast steel 2

0-0 (S0)