CORROSION CONTROL THROUGH MICRO-POROUS DRY TECHNOLOGY

Jarl-Erik Rollén

KEFA CORPORATION AB

Stockholm, Sweden

 

 

ABSTRACT

 

Rust is caused by the influence of water, oxygen and time. Air pollution and contaminants on steel surfaces speed up the corrosion process.

When corrosion is caused by condensation water, a micro-porous coating can considerably enhance traditional corrosion protection. in 4 distinct ways:

1. It delays the forming of condensation on the surface
2. It absorbs the condensation into the micro-pores during heavy condensation periods, spreads it in the porous matrix and makes it evaporate at a much faster speed than from a smooth surface. Thus it reduces the time corrosive contaminants are in contact with the surface.
3. It is anti-static and does not attract the various substances that air pollution consists of.
4. It reveals imperfections of the rust protection.

The micro-porous coat is consequently both reducing the corrosion activity time and minimizing the amount of aggressive substances contained in condensation that speed up the corrosion process.

Laboratory tests as well as full scale objects show that the life span of a traditional corrosion protection will be two to three times longer with a topcoat of a micro porous material than without one. In addition the micro-pore coat acts as an early warning system in case of defective or damaged corrosion protection.

 

 

THE CORROSION PROCESS AND CORROSION PROTECTION

 

Rust is caused by the influence of water, oxygen and time. Existing rust protecting methods can only delay the process, and in the end Nature will win the battle. 

Rust is not one homogenous chemical combination. In the first phase, water oxidizes the iron to Fe(OH)₂. Through the influence of oxygen the oxidation continues to FeO(OH) and Fe₃O₄. The end product is Fe₂O₃. (2Fe²⁺ + 4(OH)^- → 2Fe(OH)₂ → 2Fe(OH)_{2 +} O_{2 +} H₂O → Fe₂O₃  nH₂O).

Corrosion can be regarded as an electro-chemical process. Air pollution, above all chlorides and sulfates, make the water more aggressive and speeds up the corrosion process.

Contamination on the steel surface will also speed up the oxidation.

Like all chemical processes, oxidation is faster at higher temperatures.

 

The corrosion process needs water to start and the basic function of anticorrosive coatings is to prevent water from coming in contact with steel surfaces. Most anticorrosive coatings also contain rust inhibitors to repair possible weaknesses in the coating surface or damages in the protective coat. The corrosion protection must be faultless with an even thickness and contain no cracks or pinholes. Nature goes for the weakest point and even microscopic imperfections will be enough to start the corrosion process. The rust inhibitors will cure the damage temporarily, but in the long term the protective coating will deteriorate and must be replaced by a new protective barrier – often a complicated and expensive operation.

 

 

 

PROPERTIES OF MICRO-POROUS MATERIAL

 

Micro-porous materials consist of a matrix of pores with sizes varying from 10 nano meter to

10 microns. At a thickness of 40 mil the specific surface of the material is approx. 20 000 times the visible surface. Picture 1

 

The micro-porous structure gives the material certain very specific properties.

1. It insulates and delays the condensation of humid air on the steel surface.
2. It absorbs liquid condensation water and spreads it evenly in the micro-pore matrix.
3. The large specific surface speeds up the evaporation of condensation water.

 

A compound with this construction has the ability to transport liquid water by capillary forces, as well as breaking the surface tension of the absorbed water after which the water evaporates. It can be said that microspore material transforms condensate water to vapor.

 

This is the reason why the evaporation is faster from a micro-pore surface than from a smooth, painted or unpainted surface. Picture 2

 

 

THE CONDENSATION/CORROSION PROCESS WITH AND WITHOUT MICRO-PORE PROTECTION

 

Temperature, Relative Humidity (RH) and Dew point

The vapor in the air can only condense on a surface with a temperature at or below dew point. If ambient air temperature is 59°F and RH 80%, condensation starts on a surface with a temperature of 52.5°F.  Above that surface temperature there can not be any condensation.

 

Below follows a comparison between a steel surface with an anti-corrosion coating and a steel surface with a micro-porous coat.

 

Surface temperature at dew point

Steel surface: Condensation starts to appear as a thin film. Corrosion will start on unprotected spots.

Micro-pore coated surface: The micro-pore layer insulates and reduces the difference of temperature between the air and the steel surface. No condensation, no corrosion.

 

Surface temperature below dew point

Steel surface: Condensation becomes heavier. Droplets are forming on the surface and start dripping. The drop is the most corrosive form of water. The water molecules are di-polar and as they are oriented in the same direction the drop becomes a positive and a negative pole  with the anode in the middle of the drop and the cathode on the sides. The most corrosive part is where the anode is located.

Micro-pore coated surface: Depending on the temperature differential between the steel surface and the ambient air, condensation now also occurs in the micro-pore matrix or on the steel surface under the micro-pore material coat.

The micro-pores absorb the moisture and distribute it over the whole matrix where it accumulates until the conditions allow the water to evaporate. Obviously no drops can form on the steel surface.

 

Surface temperature over dew point

Steel surface  When the surface temperature is above the dew point again, the dripping of water stops. The water on the surface will gradually evaporate. Until completely dry, the steel surface is corrosive.

Micro-pore coated surface:  Due to the insulating properties of the micro-porous material this surface will be warmer than the steel surface. For that reason the absorbed condensate will start evaporating before the steel surface has reached dew point and due to the enlarged surface and the disruption of the surface tension the evaporation is very fast.

 

The micro pore material does not attract air pollution

Steel surface A cold surface attracts dust, soot and other pollution in the air. When the surface is wet the pollution sticks to the surface and especially chlorides and sulfates makes the water aggressive to the steel and its corrosion protection.

Micro-pore coated surface A warm surface does not attract particles from the air like a cold surface. The surface temperature of a micro-porous coat is always closer to the ambient temperature than a metal surface, painted or not. As a consequence the micro-pore surface will not be contaminated with air pollution. 

 

Conclusion

The time when the steel surface and its corrosion protection is exposed to condensate will at least be three times shorter with than without a micro-pore top coat.

The condensate that may occur on the steel surface under the micro-pore coat does not contain the pollutants that makes the condensate more aggressive. The risk of drops forming on the surface is eliminated. The life span of the corrosion protection will therefore be considerably extended.

 

 

SALT MIST CHAMBER TEST

 

The purpose of the test

To examine the effect of a micro-porous coat on a traditional corrosion protection of a steel plate with regard to resistance to corrosion and blistering

 

Equipment and test method

The test was carried out in an aerosol chamber as an intermittent salt fog test, where the samples were sprayed with  a salt fog solution of 0,25% NaCl and 0,25% (NH4)2SO4.

The test was carried out as a cyclic test where every 7- day cycle had the following course

 

Salt spray                                        Air temp.                                          RH

Hours                                                   °C                                                  %

 

                             Day 1                         8                                                      35                                        40>100>40

                             Day                            0                                                      35                                                  40

                             Day 3                         0                                                      35                                                  40

                             Day 4                         8                                                      35                                        40>100>40

                             Day 5                         0                                                      35                                                  40

                             Day  6                        0                                                      35                                                  40

                             Day 7                         0                                                      35                                                  40

Test period

The test was carried out in 57 weekly cycles during the period June 29 1995 - July 29 1996.

 

Samples

For the test two well known, high-build, water-based anti-corrosion paints, A and B, were used.

The paints were applied on panels of pickled, galvanized steel sheet, Sa3, and on panels of cold-rolled steel sheet. The paints were applied in coat thickness of  140 μ and 200 μ.

Every second sample was also coated with a micro-porous coat of approximately 40 mil.

All test panels had a 60 mil scratch  down to the bare metal on one part of the panel. The purpose was to study the corrosion protection in case of a lacquer damage. Picture 3

                                           

 

Result

Samples without a micro-porous top coat

1.1. All samples with a corrosion protection of 200  μ showed a very good resistance to corrosion

 and blisters on the unscratched part of the test panel.

1.2. All samples but one with a corrosion protection of 140 μ showed blisters and corrosion spots on the unscratched part of the test panel.
Most blisters were small, but the number of  blisters were dense.
 

2.1. All samples showed deep seated corrosion in the scratch.

2.2. On all samples, corrosion spread from the scratch into the coated area. The extension of corrosion from the scratch into the coated area varied from 0.2 inch to 0.44 inch on the  panels with 200 μ coats and 0.4 – 1.4 inch on the panels with 140 μ coats. 

2.3. On all samples, blisters spread from the scratch into the coated area. The spread from the scratch varied from 0.4 inch to 0.5 inch on the panels with 200 μ coats and between 0.4 inch to 1.4 inch on the panels with 140 μ coat..

 Samples with a micro-porous top coat

1.1. All samples (also the ones with 140 μ anti-corrosion coats) that had a micro-porous top coat were completely free from corrosion spots and blisters on the unscratched part of the test panel.

2.1. All samples showed deep seated corrosion in the scratch.

2.2.. There were no blisters in connection with the scratches on any of the samples that were treated with the micro-porous top coat.

2.3. Also the rust spread from the scratch was limited on the samples that were treated with the micro-porous top coat. One panel with 140 μ coat had a rust spread of 0.12 inch and one panel with 200 μ coat had a rust spread of 0.5 inch into the anti-corrosion paint.

 

Conclusion

A micro-porous top coat gives a considerable improvement to the protective properties of an anti-corrosion paint on a steel surface. A coat of 140 μ was apparently to thin for both paints, probably because it is impossible to get the exact amount of paint everywhere and because it is difficult to avoid microscopic air bubbles in the paint which form pores and craters in the coat.

The micro-porous material took care also of this problem

 

 

 

 

 

 

 

 

 

FULL SCALE EVIDENCE

 

Background

In 1992, Norwegian Rogaland Trafikkselskap built a series of ferries. All spaces exposed to moisture and condensation the entrance to the car decks, the dry tanks and the trust rooms were treated with a zinc rich epoxy primer for the corrosion protection and a micro-porous top coat. The reason for the top coat was to stop condensation water dripping down on the passengers.

 

The result

At the 5-year revision, there was a follow up on the maintenance of the painting systems. It was observed that none of the weather-exposed surfaces with a micro-pore top coat had needed any re-painting or other maintenance. All other comparable areas had been repainted several times - many areas as often as once a year. Still after 8 years, no repainting had been found necessary of the surfaces with a micro-porous top coat.

 

In 1999, when three new ferries were projected, micro-porous top coat was specified not only to stop condensation but above all for its ability to reduce maintenance costs.

 

 

A MICRO-PORE COAT ACTS AS AN EARLY WARNING SYSTEM IN CASE OF DEFECTIVE OR DAMAGED CORROSION PROTECTION

 

Water supply plants generate large amounts of water vapor that condenses on the filter tanks, pipes, pumps and other equipment. Wasserwerk, Berlin was built in the fifties and had deep-seated rust and other severe corrosion problems on equipment, steel constructions and steel roof. The premises has been re-painted several times but due to the damp environment the corrosion was back again within a year.

 

A treatment program was specified:
Roof:
1. Cleaning from soot and grease

2. Application of 40 mills of micro-pore material.

Tanks, pipes, pumps etc:

1. Blasting to SSPC-SP6.

2. Corrosion protection with good rust protection paint.

3. A micro-pore top-coat of 40 mills.

 

The system was tested on two tanks and Wasserwerk´s own contractor did the blasting and corrosion protection. The treatment looked satisfactory and after two weeks the micro-pore top-coat was applied. However, after only a few days, rust spots showed on the treated surfaces. Picture 4

 

The micro-pore top-coat was removed and it was stated that the corrosion protection had small craters and pin holes. Rust had been solved by the condensation water and spread in the micro-porous matrix. The actual rust spots had a size of a needle tip and were hardly detectable, but the micro-pore material spread it out to a size of a some hundred mil.

 

The corrosion protection and application of the micro-pore topcoat was re-done by a certified contractor. After checking the surfaces for two months the rest of the premises were treated as specified. During and two weeks after the application three large dehumifiers were running to allow the micro-pore coat to dry.

     

The surfaces are completely spotless after 29 months. The coated surfaces are warmer than they were before and there is no condensation water on the surfaces. Earlier maintenance jobs lasted only a year before rust spots were showing on the surfaces. Picture 5

An identical hall in the same building, that has not been treated, shows heavy condensation on steel constructions and equipment and the ambient air seems colder and more humid.

 

 

CORROSION UNDER INSULATION

 

Corrosion under thermal insulation is a severe problem. Theoretically and under very controlled conditions, a vapor barrier will prevent moisture from penetrating the insulation and condense, but in practice it is more or less impossible. When the vapor pressure of the ambient air is higher than the vapor pressure in the insulation, vapor will penetrate and condensate inside the insulation. This condensate runs down to the steel surface and corrosion starts. When the vapor pressure of the ambient air goes down again the condensate is in liquid form and can not get through the vapor barrier. The condensate accumulates inside the insulation. Therefore practically all insulated pipes and similar constructions are rusting and the inspection and maintenance costs are high.

 

The remedy is a micro-pore coat on the steel surface under the insulation. When condensate water runs down on the steel surface, the micro-pore material absorbs the water, stores it in the pores until the ambient vapor pressure sinks. Then the water is released in the form of vapor and can consequently get through the imperfections in the vapor barrier back into the ambient air.

 

 

CONCLUSION

 

 

A top coat of micro-porous material can not work without a proper corrosion protection.

It will immediately disclose imperfections in the corrosion protection.

The micro-porous material will shorten the wet corrosive time and thereby prolong the effective life of the corrosion protection.

 

It can be used under a thermal insulation to prevent rust and other corrosion on the steel surface.

Experience shows that a topcoat of micro-porous material extends the lifespan of a corrosion protection more than two times.

Sample category	Type of panel	Anti-corrosion  paint	Coat thickness	micro-porous top coat
1.1 1.2	Pickled, galvanized  steel sheet	A	200 μ 140 μ	No
2.1 2.2	Pickled, galvanized  steel sheet	A	200 μ 140 μ	Yes
3.1 3.2	Pickled, galvanized  steel sheet	B	200 μ 140 μ	No
4.1 4.2	Pickled, galvanized  steel sheet	B	200 μ 140 μ	Yes
5.1 5.2	Cold rolled steel sheet	A	200 μ 140 μ	No
6.1 6.2	Cold rolled steel sheet	A	200 μ 140 μ	Yes
7.1 7.2	Cold rolled steel sheet	B	200 μ 140 μ	No
8.1 8..2	Cold rolled steel sheet	B	200 μ 140 μ	Yes

Picture 3

Picture 4. Rust spots exposed by micro-porous material
Picture 5. Wasserwerk Berlin, 26 months after treatment.