Corrosion Protection by Ts

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Budapest University of Technology and Economics

April 28th of 2015

CORROSION PROTECTION OF STEEL

STRUCTURES

Construction Technology

Student: Tuane de Souza Schuenke

Neptun Code:BE6WGQ

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Table of Contents: 1. Introduction .................................................................................................................................. 3

2. Aspects of Corrosion Protection of Steel Structures: ...................................................... 4

2.1. The Corrosion .......................................................................................................................... 4

2.2. Surface Preparation: .............................................................................................................. 5

2.2.1. Hand and Power Tool Cleaning: ..................................................................................... 5

2.2.2. Abrasive Blast Cleaning: .................................................................................................. 6

2.2.3. Flame Cleaning: .................................................................................................................. 6

2.2.4. Acid Pickling: ....................................................................................................................... 6

2.2.5. Removal of Soluble Iron Corrosion Products: ........................................................... 7

2.2.6. Surface Dust: ....................................................................................................................... 7

2.2.7. Additional Surface Treatments: ...................................................................................... 7

2.3. Methods of Corrosion Protection: ..................................................................................... 8

2.3.1. Paint Coatings: .................................................................................................................... 8

2.3.1.1. Primers: .............................................................................................................................. 8

2.3.1.2. Undercoats (Intermediate Coats): .................................................................................. 8

2.3.1.3. Finish Coat: ........................................................................................................................ 9

2.3.1.4. Stripe Coat: ........................................................................................................................ 9

2.3.1.5. The Paint System: ............................................................................................................ 9

2.3.1.6. Main Types of Paint and Their Properties: ................................................................... 9

2.3.1.7. Prefabricated Primers: ................................................................................................... 10

2.3.1.8. Application of Paint Coatings: ....................................................................................... 10

2.3.2. Metallic Coatings: ............................................................................................................. 11

2.3.2.1. Hot-dip Galvanizing: ....................................................................................................... 11

2.3.2.2. Thermally Sprayed Metal Coating: ............................................................................... 12

2.3.2.3. Other metallic coatings: ................................................................................................. 13

2.3.2.4. Bolts, Nuts and Washers: .............................................................................................. 13

2.4. Influence of design on corrosion: .................................................................................... 14

2.5. Choice of the Protection:.................................................................................................... 14

2.6. Appropriate Specifications: ............................................................................................... 15

2.7. Quality Control: ..................................................................................................................... 15

2.8. Health and Safety: ................................................................................................................ 15

3. Conclusion: ................................................................................................................................ 16

4. References:……………………………………………………………………………………….…………………………………17

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1. Introduction

Construction is the most important steel using industry, representing more than

50% of world steel production. The majority of this steel is used for reinforced

concrete. But the use of steel material for structures has many advantages, which

include great strength to weight ratio, ductility, predictable material properties, speed

of erection, ease of repair, adaptation to prefabrication, good fatigue strength, steel

buildings can be easily expanded and have possibility of reuse the material.

The disadvantages of steel as a structural material include the higher general

cost, the needing of adequate fireproofing, susceptibility to bucking and the possibility

of corrosion of the material. The corrosion can be really dangerous for some

structures, depending mostly of the environment to which the structure is exposed. It

requires an adequate protection and maintenance.

The key to success involves recognizing corrosivity of the environment and

defining the appropriate coating and its specifications. If the steel is in a dry heated

inside interior, the risk of corrosion is low and no protective coating is necessary.

However, if the structure is exposed on an aggressive environment, a sophisticated

protection may be required and also a maintenance plan should be organized to

guarantee the adequate lifetime of the structure.

The optimum protection treatment combines surface preparation, suitable coating

materials, required durability and minimum cost. The aspects of protection,

environmental effects and techniques are the subject of this work and will be

presented during this paper.

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2. Aspects of Corrosion Protection of Steel Structures:

2.1. The Corrosion

The Corrosion of steel is a electrochemical reaction between a material and its

environments, resulting in deterioration of the material and its properties. It requires

presence of oxygen and moisture. The presence of pollutes can accelerate the

process. The iron in the steel is oxidized to produce rust, as we can see in the

equation below:

Fe + 3O2 + 2H2O = 2Fe2O3H2O (Steel + Oxygen + Water = Rust)

The progress of corrosion depends of the micro-climate surrounding the

structure, mostly the time of wetness and atmospheric pollution level. We can classify

the environment according to the corrosivity categories and obtain some indications

about the rate of corrosion in which case. It’s important to recognize the corrosivity of

the environment which the structure will be exposed to define the appropriate coating

specifications.

Below, we can see some examples of corrosion of steel structures:

1. Corrosion in Devil's Elbow Bridge. Source:http://www.rollanet.org/~conorw/cwome/article51&52combined.htm

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2. Corrosion in the gusset plates of Mercier Bridge. Source: http://www.paintsquare.com/news/?fuseaction=view&id=7219

2.2. Surface Preparation:

The preparation of the surface is an essential process before the application of

any coating. This step is considered the most important factor to the success of the

corrosion protection system because the performance of the coating it’s related to the

properly adherence to the substrate material.

The selection of the type of surface preparation depends on the type of paint

used, the condition of the surface, economic considerations and practical limitations

(accessibility, time,…). There are several methods of preparation which will be

presented below.

2.2.1. Hand and Power Tool Cleaning:

The use of hand tools (scrapers and wire brushes) is considered ineffective in

removing mill scale and rust. Power tools are a little more effective, but usually these

methods are used when it is not possible to clean by abrasive blasting.

3. Hand wire brush. Source:

http://www.steelconstruction.info/Surface_ preparation#Initial_surface_condition

4. Rotary wire brush. Source:

http://www.steelconstruction.info/Surface_ preparation#Initial_surface_condition

5. Rotary grinder. Source:

http://www.steelconstruction.info/Surface _preparation#Initial_surface_condition

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2.2.2. Abrasive Blast Cleaning:

This method is the most important used for remove mill scale, rust and

corrosive salts. It works by the continuous impact of abrasive particles at high

velocities on the steel surface, using a jet stream of compressed air or by centrifugal

impellers.

The particle size of the abrasive affects the rate and efficiency of the cleaning.

Fine grades are efficient for new steel work while coarse grades are more used for

heavily corroded surfaces, for example. Sometimes is necessary a mixture of grades

for a better preparation.

Many local air pollution control agencies may restrict dry abrasive blasting

because of the dust emitted during the cleaning. In this case, it is possible to add

water to the abrasive to meet the air pollution control regulations. A rust inhibitor

needs to be added to the water to prevent rusting before the application of the

protection. In the figure 6 we can see a man executing the abrasive blast cleaning.

6. Abrasive Cleaning. Source: http://www.chescoat.com/abrasive_blasting.html

2.2.3. Flame Cleaning:

In this method, an oxy/gas flame is passed over the steel surface. Due to

differential expansion between the scales and the steel, the rust scales flake off with

the heating. All the loose rust can be removed by scraping, wire brushing and dust

removal. This method is not so effective to remove all the dust. Also, this is an

expensive method and it can damage coatings in the reverse side of the structure.

2.2.4. Acid Pickling:

The structure is submerged in suitable inhibited acids that dissolve the mill

scale and the rust without attacking the steel. This cleaning can be 100% effective.

This method usually is used for structural steel intended for hot-dip galvanizing.

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2.2.5. Removal of Soluble Iron Corrosion Products:

Depending on the condition of steelwork, it could contain contaminants within

the pits that may not be removed by dry blast cleaning process. We can use the wet

abrasive blast cleaning (already mentioned at 2.2.2) with this propose. Another

alternative is the ultra-high pressure jetting which uses lower volumes of water

comparing the previous method and do not need abrasive particles (but they can be

used in small amounts). It can remove high proportion of soluble salts.

7. Wet abrasive blast cleaning. Source:

http://www.alibaba.com/product-detail/Aqua-Jet-Wet-Abrasive-Blasting-System_123201793.html

8. Ultra-high pressure jetting. Source:

http://www.steelconstruction.info/ Surface_preparation#Initial_surface_condition

2.2.6. Surface Dust:

The blast cleaning operation produces dust and debris that must be removed

from the surface. It can be removed by mechanical brushes, air blowing, sweeping

and vacuum cleaning. The presence of not visible residual dust can be checked

using a pressure sensitive tape on the surface.

2.2.7. Additional Surface Treatments:

After abrasive blast cleaning, it is easy to search for imperfections and surface

alterations during fabrication process. Some imperfections will not influence the

effectiveness of the coating system, but others may affect and they need to be

removed. Welding process can create uneven surfaces or sharp projections that can

damage the coating.

The meeting between two surfaces can also be a problem. In case of sharp

edges coating will not cover it properly. Usually smooth the corner to a radius of

around 2mm is enough. Chamfering to 45° can also solve the problem but it is

difficult to avoid sharp edges using hand tools for this procedure.

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2.3. Methods of Corrosion Protection:

The corrosion protection can be divided in paint coatings and metallic

coatings. Their types, characteristics, specifications will be shown below.

2.3.1. Paint Coatings:

This protection works by coating the steel with a protection, isolating the

material from its environment and consequently forming a solid barrier against

oxygen, water and pollutes. The protection is better when the permeability is lower.

The corrosion in the material will not occur if the barrier is intact. However, if the

barrier is disrupted the corrosion will begin.

Paints are composed by three components: pigments, binder and solvent. The

pigments provide color, opacity, film cohesion and can provide also corrosion

inhibition. The binder is the film forming component in the paint. The solvent is used

to facilitate the application of the paint, dissolving the binder.

The paint is applied to the surface and after the evaporation of the solvent a

“dry” film is formed in the surface. Usually the thickness of the film is proportional to

the protection of the painting. The paint coating systems consist of primer,

undercoat(s) and finish coats. But there are available coat systems that combine

primer and finish coat. They are applied one on the top of another and each of them

has a specific function.

2.3.1.1. Primers:

The function of the primers is wet the surface and provide good adhesive for

the further coats. It is applied directly onto the cleaned steel surface or after the

sealed metal coating (duplex systems). For primers applied directly onto the surface,

it should provide corrosion inhibition. The primers are divided in two basic types:

Primers pigmented with metallic elements anodic to steel and primers relying on the

high adhesion and chemical resistance properties of the binder.

The first one, in case of damage in the coating (exposing the substrate), the

primer will corrode sacrificially instead of the steel. It happens until the anodic

material is exhausted. The most used primer of this type is the zinc-rich primer. The

second type of primers provides a good adhesion sufficient to prevent under-rusting

in case of damages in the coating. The necessary adhesion is obtained only if the

surface was cleaned adequately. Two-pack epoxy primers are typical in this case.

These primers may contain inhibitive pigments to interfere with corrosion process.

2.3.1.2. Undercoats (Intermediate Coats):

The undercoat is used to increase the thickness of the coating and provide

more protection. It can contain pigments which decrease the permeability of the

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coating to oxygen and water, or laminar pigments that reduce (or delays) the

moisture penetration and improves tensile strength.

2.3.1.3. Finish Coat:

This coat provides appearance and surface resistance of the system. It will

also provide first line protection against weather exposure, sunlight and

condensation.

2.3.1.4. Stripe Coat:

Additional coat applied to welds, fasteners and external corners. They will

build a satisfactory thickness in these places where the paint tends to contract. When

they will be applied depends of specifications, as the type and number.

2.3.1.5. The Paint System:

The different coats need to be compatible with each other. All paints of the

same system should be obtained from the same manufacturer and used according

recommendations. After the finish of the coat system, it is important to measure the

thickness of the dry film. Individual values less than 80% of the nominal thickness are

not accepted. Values between 80% and 100% are accepted if the overall average is

equal or greater than the nominal value. The system should respect the minimum

thickness and an over application can result in high stresses and premature failure of

the system.

9. Protective paint system. Source: http://www.npl.co.uk/upload/pdf/steelwork.pdf

2.3.1.6. Main Types of Paint and Their Properties:

2.3.1.6.1. Air drying paints:

These materials dry and form a film by an oxidative process, involving

absorption of oxygen. The formed film has limited solvent resistance and poor

chemical resistance, and this process forms relatively thin films (typically 40μm).

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2.3.1.6.2. One pack chemical resistant paints:

Film formation requires only solvent evaporation. The formed film remains

relatively soft and has poor solvent resistance but good chemical resistance. It forms

moderately filme thickness (around 50-75μm).

2.3.1.6.3. Two pack chemical resistant paints:

Two different components (base and curing agent) are mixed before use

occurring a chemical reaction. After the mixture, the paint must be applied before the

“pot time”, the maximum time of application specified by the manufacturer. The

reaction continues after the application and the evaporation of the solvent. The film

formed can be very hard and has good solvent and chemical resistance. In this paints

it is possible to use liquid resins instead of solvent.

2.3.1.7. Prefabricated Primers:

These primers are used to maintain the reactive blast cleaned surface in a rust

free condition until the final painting can be performed. Some special requirements

should be fulfilled in this kind of primer. Some examples are the etch primers, epoxy

primers, zinc epoxy primers and zinc silicate primers.

2.3.1.8. Application of Paint Coatings:

The method of application of the paint affects the quality and durability of the

coating. Some methods include application by brush, roller, conventional air spray

and airless spray. Brushing is the simplest method of application but also the slower

and more expensive one. This method provides a better wetting of the surface, can

be easily applied in small areas with less wastage and contamination of

surroundings. The roller is much faster than brushing but demands suitable

rheological properties of the paint and can only be used for flat areas. The roller

application can ride over high spots and miss edges and it is not adequate for

corners and bolts.

In structural steel, paint coatings are usually spray applied. In air spray, the

paint is atomised by mixing it with a stream of compressed air. The spray nozzle and

air pressure must be adjust by the operator according to the requirements and

compositions. This applications is faster than brushing and rolling but has a higher

wastage. For airless spray, the paint is hydraulic compressed and, on release

through a small orifice in a spray gun, it is atomised and projected onto the surface.

We can change the paint consistencies changing the shape and size of the orifice

and hydraulic pressure. The equipment is more expensive than conventional spray

but the wastage is lower.

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10. Brushing. Source:

http://www.steelconstruction.info/Paint_coatings#Composition_of_paints_and_film_formation

11. Rolling. Source:

http://www.steelconstruction.info/Paint_coatings#Composition_of_paints_and_film_formation

12. Air spray. Source:

http://www.steelconstruction.info/Standard_corrosion_protection_systems_for_buildings

13. Airless spray. Source:

http://www.steelconstruction.info/Paint_coatings#Composition_of_paints_and_film_formation

The temperature and humidity can affect the application of paint coatings.

These conditions can be more easily controlled in shops. Paints should not be

applied if there is condensation on the steel surface or in certain relative humidity of

the atmosphere.

2.3.2. Metallic Coatings:

Steel substrate is coated with a layer of zinc, a zinc/aluminum alloy, a

zinc/silicon alloy or pure aluminum using different processes. There are four

methods: Hot-dip galvanizing, thermal spraying, electroplating and sherardizing. The

last two are not used for structural steelwork.

2.3.2.1. Hot-dip Galvanizing:

In this process the steel component is immerged in a bath of molten zinc about

450°. This is the most common method of applying a zinc coat into structural steel.

The first step of the process is remove oil or grease from the surface using suitable

degreasing agents. Then the surface is cleaned by acid pickling and sometimes blast

cleaning. The steel is submerged in a fluxing agent to ensure good contact between

the steel and the zinc. Then the steel is immersed into the molten zinc. The steel

reacts with the zinc and form a series of zinc/iron alloys integral with the surface.

After removing the piece from the bath, a layer of pure zinc is applied on the top.

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The thickness of the surface will vary according to the size and thickness of

the piece and the surface preparation. The resulting coating is durable, tough,

abrasion resistant and provides sacrificial protection in case of damages. The size of

the pieces to be treated is limited.

During the design of the structure components, it is important to take

galvanizing process into account, because some special cares are necessary to

avoid further problems. After the process, the steel can be used without other

protection. Paint coatings can be applied for esthetic reasons or to provide extra

durability. The combination of paint and metal coating is called duplex coating. In the

figures below we can see some steps of the process.

14. Steps of hot-dip galvanizing process. Source: http://archive.galvanizeit.org/sd-seminar/what-is-hot-dip-galvanizing

15. Steel piece been submerged in the zinc bath. Source:

http://www.weland.se/?ID=YTBEHANDLING1&sLang=en-gb

16. Piece after the whole process. Source: http://www.tube-mill.com/products/Hot-dip-galvanizing-tube-pipe-line.html

2.3.2.2. Thermally Sprayed Metal Coating:

This method is another alternative of applying metallic coating to the steel

surface. It is possible to use zinc or aluminum. The metal (powder or wire form) is

thermally sprayed at the steel surface by a special spray gun containing a heat

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source (oxygas flame or electric arc). No alloying occurs, the coating it is formed by

overlapping platelets of metal and its porous. The porous must be sealed by an

organic coating which penetrates into the surface.

This method can be applied at the site or in the shops and there is no

limitation of size for the steel pieces. Usually this process is more expansive than hot-

dip galvanizing. In this method is also possible to apply a paint coating after the

metallic protection. In the figures below we can see the surface after the application

(16) and an example of application (17).

17. Surface after the application of thermally sprayed metal coatings.

Source: http://www.advanced-coating.com/english/spraying.htm

18. Application of thermally sprayed metal coating. Source:

http://www.steelconstruction.info/Metallic_coatings

2.3.2.3. Other metallic coatings:

The electroplating method and the sherardizing method are used for fittings,

fasteners and other small items. Since they are not used for structural steelwork they

are not specified in this project.

2.3.2.4. Bolts, Nuts and Washers:

These elements need to be protected to at least the same level as the main

members. Short-term protection can be applied but it is recommended to apply the

full coating system after assembling.

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2.4. Influence of design on corrosion:

The design of a structure can influence in the corrosion protection applied to

the surface. Small pieces are more difficult to protect. It is important to consider the

accessibility for coating application and maintenance, avoid accumulation of water

and dirt and be careful with the management of contact with other materials. The

figure below (19) shows some recommendations for better results of the corrosion

protection.

19. Recommendations for the design. Source: http://www.infosteel.be/publicaties/guide-to-protection-of-steel-against-corrosion.pdf

2.5. Choice of the Protection:

The choice of the more adequated protection is affected by several factors, including

expected life of the structure, feasibility of maintenance, the environment, size and shape of

structural members, shop treatments facilities available to the fabricator or sub-contractor,

site conditions and the cost.

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2.6. Appropriate Specifications:

It is important to have a well-prepared document providing clear and precise

instructions to the contractor. This specification should be done by a technical expertise and

should contain a logical sequence and all the necessary information for the execution of the

protection work.

The most important items in a specification are the method of surface preparations,

metal dressing to remove sharp edges and steel contamination, maximum interval between

surface preparation and application of coating, the types of coating that should be used and

their standards, the application methods to be used, number of coats and intervals of

application, wet and dry film thickness, where each coat should be applied and their

application conditions, details of treatment for connections and procedure for damages.

2.7. Quality Control:

The inspetions ensure that the requirements are being met. Inspetion of the materials,

the processes and procedures are quite important since a mistake not identified can

compromise the next steps of the coating and reduce the quality of the protection system.

2.8. Health and Safety:

There are some special cares to be taken account during the operations process.

Certain materials and components have restrictions of use because of their toxicity. Noise

produced by some processes (blast cleaning for example) should be below the level that

would damage hearing. Explosion hazarads must be avoided and operators have to use

protective equipaments according to their function.

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3. Conclusion:

Nowadays, there are special technique developed with steel which not require

corrosion protection (stainless steel, weathering-resistance), but these materials are

more expensive and are economical feasible only in special cases. So, the coating as

a corrosion protection is the most important method utilized to avoid the corrosion of

the steel and its consequences.

The corrosion protection of steel structures is an essential factor to guarantee

the appropriate lifetime of the structure. The choice of the appropriate technique is

really important, besides the application of the coating and preparation of the surface

also play an important role to the success of the protection, as explained in this

project.

The corrosion protection which will be applied in the structures must be

thought since the design of the structure. The engineers involved in the project

(design, execution…) must always work together to obtained better results.

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4. References:

http://www.steelconstruction.info/Corrosion_protection

http://user.engineering.uiowa.edu/~swan/courses/53086/Corrosion_Protection.pdf

http://www.bd.gov.hk/english/documents/code/SUOS2011.pdf

http://www.galvanizeit.org/hot-dip-galvanizing/why-specify-galvanizing/corrosion-

protection

http://www.infosteel.be/publicaties/guide-to-protection-of-steel-against-corrosion.pdf

http://www.npl.co.uk/upload/pdf/steelwork.pdf

http://www.steelconstruction.info/Surface_preparation#Initial_surface_condition

http://www.steelconstruction.info/Paint_coatings#Composition_of_paints_and_film_fo

rmation

http://www.steelconstruction.info/Metallic_coatings

http://www.szs.ch/user_content/editor/files/Downloads_Stahlwerkstoffe/metallic%20c

oated%20steel.pdf

http://resource.npl.co.uk/docs/science_technology/materials/life_management_of_ma

terials/publications/online_guides/pdf/protection_of_steel_bridges.pdf