PPT.pptx

CORROSION PROTECTIONBasic corrosion theory and protection methods

Corrosion & Corrosion Control

• What is Corrosion • How/Why Does Corrosion Occur

• Corrosion Costs • Forms of Corrosion • Corrosion Control Methods Hot-dip Galvanizing (HDG) Process Coating Characteristics Performance in Corrosive Environments  • Galvanized Steel in Action

What is Corrosion?

Corrosion

• The chemical or electrochemical reaction between a material and its environments that produces a deterioration of the material and properties.

The Galvanic Series

• ZINC – Anode• STEEL – Cathode• This arrangement of metals determines what metal will be

the anode and cathode when the two are put in a electrolytic cell (arrangement dependent on salt water as electrolyte).

Bare Steel Corrosion

• Microscopic anodic and cathodic areas exist on a single piece of steel.

• As anodic areas corrode, new material of different composition is exposed and thus has a different electrical potential

Forms of Corrosion

• General

Identified by uniform formation of corrosion products that causes a even thinning of the substrate steel

• Localized

Caused by difference in chemical or physical conditions between adjoining sites

• Bacterial

Caused by the formation of bacteria with an affinity for metals on the surface of the steel

• Galvanic/Dissimilar Metal

Caused when dissimilar metals come in contact, the difference electrical potential sets up a corrosion cell or a bimetallic couple

Corrosion CostsIndirect Cost • Catastrophe

Public safety, property damage, environmental contamination

Natural Resource

Waste production increased energy consumption

 • Public Outcry

Traffic, inconvenience

Direct Costs

NACE, CC Technologies, & FHWA jointly produced a report in 2001 detailing the costs of corrosion

• $276 billion USD annually

• 3.1% of US GDP (1998)

Methods of Corrosion Control• Barrier Protection

Provided by a protective coating that acts as a barrier between corrosive elements and the metal substrate

• Cathodic Protection

Employs protecting one metal by connecting it to another metal that is more anodic, according to galvanic series

 • Corrosion Resistant Materials

Materials inherently resistant to corrosion in certain environments

Barrier Protection• Paint • Powder Coatings• Galvanizing

Cathodic Protection• Impressed Current• Galvanic Sacrificial Anode• Galvanic Zinc ApplicationZinc Metallizing Zinc-rich Paints Hot-dip Galvanizing

• Impressed Current• External source of direct current power is connected (or

impressed) between the structure to be protected and the ground bed (anode)

•  Ideal impressed current systems use ground bed material that can discharge large amounts of current and yet still have a long life expect

Galvanic Sacrificial Anode• Pieces of an active metal such as magnesium or zinc are

placed in contact with the corrosive environment and are electrically connected structure to be protected

Galvanic Zinc Application

Zinc Metallizing (plating)

 • Feeding zinc into a heated gun, where it is melted and sprayed on a

structure or part using combustion gases and/or auxiliary compressed air

Zinc-rich Paints

 • Zinc-rich paints contain various amounts of metallic zinc dust and are

applied by brush or spray to properly prepared steel

Hot-dip Galvanizing

 • Complete immersion of steel into a kettle/vessel of molten zinc

• Zinc Metalizing

• Zinc Rich Paints

• Hot Dip Galvanizing

Hot-dip Galvanizing Process• Surface Preparation• Galvanizing• Inspection

Surface PreparationZinc-iron metallurgical bond only occurs on clean steel

Degreasing

• Removes dirt, oils, organic residue

Pickling

• Removes mill scale and oxides

Fluxing

• Mild cleaning, provides protective layer

Galvanizing• Steel articles are immersed in a bath of molten zinc (≈ 830

F)

• > 98% pure zinc, minor elements added for coating properties (Al, Bi, Ni)

• Zinc reacts with iron in the steel to form galvanized coating.

Inspection• Steel articles are inspected after galvanizing to verify

conformance to appropriate specs.

• Surface defects easily identified through visual inspection.

• Coating thickness verified through magnetic thickness gauge readings.

FIRE PROTECTION OF STEEL STRUCTURES

There are four common methods of fire protecting structural steelwork

• Board based systems• Intumescent coatings• Sprayed fire protection systems• Concrete encasement or filling

Board based systems

• Board based systems are used to form rectangular encasements around steel members, such as internal beams and columns.  Paint or other finishes can be applied directly to the boards.

• The level of fire resistance achieved depends upon the type and the thicknesses of the boards used and upon the method of attachment.

• There are broadly two families of board protection, lightweight and heavyweight. Lightweight boards are typically 150-250kg/m³ and are not usually suitable for decorative finishes.

• They are typically used where aesthetics are not important and are cheaper than heavyweight equivalents. Heavyweight boards are usually in the range 700-950kg/m³ and will generally accept decorative finishes. They are typically used where aesthetics are important.

• Both types of board may be used in limited external conditions but the advice of the manufacturer should be sought. Detailed guidance on the installation of board protection systems is available from the Association for Specialist Fire Protection

Intumescent coatings

• Intumescent coatings may be brushed or sprayed onto steelwork rather like paint. The materials expand when subjected to fire and form an insulating foam. Intumescent coatings can achieve up to 120 minutes fire resistance, and are used mostly on exposed steelwork.

• Intumescent coatings are paint like materials which are inert at low temperatures but which provide insulation as a result of a complex chemical reaction at temperatures typically of about 200-250°C. At these temperatures the properties of steel will not be affected. As a result of this reaction they swell and provide an expanded layer of low conductivity char.

• Intumescent coatings can be divided into two broad families: thin film and thick film. Thin film materials are either solvent based or water based and are mainly used for cellulosic fire conditions. Thick film intumescent coatings were originally developed for the off-shore and hydrocarbon industries but have been modified for use in buildings.

Sprayed fire protection systems

• Sprayed fire protection systems are usually based upon cementitious materials and are applied directly onto the surface of steelwork. They are generally low cost, but cannot receive finishes owing to their coarse uneven texture. Sprayed materials tend to be used where steelwork is concealed or where appearance is unimportant. Fire resistance is similar to that of board based materials.

• Spray protection is extensively used in the United States but is less common in the United Kingdom. It has the advantage that it can be used to cover complex shapes and details and also that costs do not increase significantly with increases in protection thickness.

• Sprays are not suitable for aesthetic purposes. Also, application is a wet trade and this may have impacts on other site operations. Allowance may have to be made in costing for the possible requirement for prevention of overspray

Concrete encasement or filling

• Structural Hollow Sections (SHS) can be fire protected by filling with reinforced concrete. Concrete filled structural hollow sections can achieve 120 minutes fire resistance.

• Multi-storey frames requiring 30-60 minutes can have 40% of the floor beams unprotected by following the recommendations of a special design guide.

• Until the late 1970s, concrete was by far the most common form of fire protection for structural steelwork. However the introduction of lightweight, proprietary systems such as boards, sprays and thin fill intumescent coatings has seen a dramatic reduction in its use. 

•  The principal advantage of concrete is durability. It tends to be used where resistance to impact damage, abrasion and weather exposure are important e.g. warehouses, underground car parks and external structures. The principal disadvantages are:

• Cost - compared to lightweight systems;• Space utilisation (large protection thicknesses take up

valuable space around columns)• Weight.

Protection thicknesses

• The section factor of a particular steel section is its surface area per unit length divided by its volume per unit length (A/V). This parameter defines how quickly a steel section will heat up when subjected to fire. The section factor for a member with box protection is lower than that for a member with profile protection, and hence box protected steelwork heats up more slowly and requires less protection.

• Typical spray or board thicknesses for a column in a multi-storey building are as set out in the table below.

• Typical spray or board thicknesses based on 254UC x 89 kg/m column in a multi-storey building.

Fire resistance(minutes) Profile Protection(mm) Box Protection   (mm)

30 10 12

60 18 15

90 24 20

120 30 25

THANK YOU AND MABUHAY!