9 corrosion prevention design material select

8/12/2019 9 Corrosion Prevention Design Material Select

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CORROSION

PREVENTION

BY

DESIGN


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INTRODUCTION

A good design at the blackboard is no

more costly than a bad design a bad

design is always more expensive than a

good design in reality.

Design has a critical role to play in the

service life of components.

The important point is that the designers

must have an understanding and

awareness of corrosion problems.


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More attention is paid to the selection ofcorrosion resistant materials for a specificenvironment, and a minimal considerationis given to design, which leads toequipment failure.

This has been a common observation indesalination plants in the Gulf region.

We will highlight how corrosion could beprevented by adopting good designpractices.


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SERVICE LIFE OF EQUIPMENT

Selection of a corrosion resistant material for theenvironment is a prerequisite to a good design.

Materials and designare complimentary to each

other and neither of the two can be ignored.

The following factors influence the service life ofequipment:

1) Environments and geographic location2) Selection of materials

3) Maintenance

4) Corrosive environment arid velocity of flow

5) Design

6) Feature promoting corrosion7) Bimetallic connection.


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CAUSES OF FAILURES IN DESIGN CONTEXT

A good engineering design should provide a maintenance-free service, satisfy the end user, and

provide a maximum return on capital in a shortest return period.

However, there are several areas related to failure as show below.

1) Breakdown of protective system.

Many protective surface treatments, such as coating and welding,may not be very effective because of the presence of surfaceirregularities, voids, surface porosity, undercuts, and general surfaceroughness.

The surface heterogeneities act as moisture traps and cause the

damage.

2) Poor fabrication.

Factors, such as improper welding, excessive cold working andexcess machining lead to failure.


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3) Lack of accessibility.

In complex systems, machinery, and components, there might beinaccessible areas due to lack of design insight where it may not bepossible to carry out the corrosion protection measures.

Interiors of car doors are examples which are subjected to intensivelocalized corrosion.

Figure 8.3 shows a design which provides adequate air circulationarid spraying accessibility.

4) Structural heterogeneity in materials.

Joining similar materials with structural differences, such asdifferences in thermo-mechanical processing, grain size, number ofimpurity elements, grain boundary segregates, may cause deviationfrom the performance expected.

5) Operating conditions.

Factors, such as temperature, pressure, arid velocity, influence theservice life if allowed to exceed the prescribed limits.


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Consideration of Corrosive Environment

The metal or alloy must have a proven compatibility to

the corrosive environment.

For instance, stainless steel (SS) 316 with 2% Mo is abetter material for seawater service than SS 304 without

molybdenum.

Brass, bronze and copper based alloys are highlydesirable for salt water transportation. However, they arevulnerable for an environment containing ammonia

frequently encountered in agriculture.

A good design to prevent corrosion must be compatiblewith the corrosive environment.


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Major Contributors to Corrosive Environments toConsider in Design:

1) Temperature.- Temperatures slightly in excess of 50C are observed

in several GCC countries.

- High temperatures in combination with high humidityproduce an accelerating effect on corrosion.

2) Humidity.

Corrosion progresses fast when the relative humidityexceeds 75%.

3) Rainfall.

Rain can be beneficial or harmful:- Excess rainfall washes corrosive materials and removes dirt,

debris and other deposits which may initiate corrosion.

- Scanty rainfall may leave water droplets on the surface and leadto corrosion as salt is present in the air.

- Frequency of rainfall contributes to humidity.


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4) Pollution.

Coastal Regions

Atmosphere contains sodium chloride particles plusothers, e.g. sulfur dioxide, sulfurous and sulfuric acid;among the worst offenders regarding corrosion.

Sources: power stations, refineries, chemical andmetal manufacturing plants. They are abundant in GCCcountries.

Desert

Abundance of sand particles accelerates corrosionbecause of the hygroscopic nature of some constituentsof sand particles.

Cold Climate

Presence of sodium chloride which is extensivelyused in deicing of roads in Europe and North America.

Use of small amounts can induce high levels of

corrosion in road vehicles.


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The atmosphere may also contain other pollutants, suchas carbon monoxide, nitrogen oxides, hydrocarbons,etc.

5) Proximity to Sea.

Seawater is considered to be equivalent to a 3.5%solution of sodium chloride.

There is abundance of chloride in the marineenvironment and in industrial zones located in marineenvironment.

A cumulative corrosive effect is caused by both chloride

and sulfur dioxide. Chlorides can absorb moisture at lowrelative humidities.

Saturated NaCI solution is in equilibrium with a relativehumidity of 78%.

Saturated ZnCl2solution is in equilibrium with only 10%.


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Specific Design Items

Design items which affect the rate of corrosion includethe following:

1) Stresses acting on the materials in service.

2) Relative velocity of the medium and obstacles toflow.

3) Bimetallic contacts.4) Crevices.

5) Riveted joints.

6) Spacing for maintenance.

7) Drainage and directional orientation of loop.8) Joints to avoid entrapment.

9) Sharp corners.

10) Non-homogeneous surface.


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CONSIDERATION OF AREAS REQUIRING ATTENTION

AT THE DESIGN STAGE

The following are the areas which require attention tominimize corrosion:

1. Bimetallic contacts 2. Faying surface 3. Crevices

4. Moisture traps 5. Water traps 6. Welds

7. Inaccessibility 8. Areas of condensation

9. Fluid movements

10. Metals in contact with moisture absorbent materials

11. Features which reduce the paint thickness

12. Oil, grease, rust patches

13. Joints: threaded, riveted, screwed14. Closed sections and entrapment areas

15. Mechanical factors 16. Corrosion awareness

Effects of some factors stated above on design are brieflydescribed.


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To avoid retention of water (moisture) layer on channel surface

in case of bi-metallic contact


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Figure 8.24 shows the

suitability of soldering over

threaded joints.

Figure 8.25 shows

comparison of spot welded

joints and riveted. The figure

also illustrates why welded

joints are preferable to riveted.


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FACTORS CONTRIBUTING TO POOR DESIGN

1- Ignoring Specifications

There is a general trend to use PVC pipes in gas aridwater distribution systems.

Concrete pipes have been used widely for water mains.

There are specifications on soil compaction, thepressure the pipes can withstand, and the composition ofsoils.

Non-adherence of these specifications lead to serious

failures.

Copper pipes are joined in several instances with steelpipes without proper insulation and coatings which leadsto service problems of galvanic corrosion.


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2- Putting Dissimilar Pipes in the Same Trench

Pipes of different materials, such as copper, steel mild

steel and galvanized iron are often buried very close toeach other in the same trench without any concern forgalvanic corrosion.

The copper pipe is coated and insulated to minimize

galvanic corrosion. The mild steel pipe may be protectedby a galvanic anode but this is not cost effective.

Note:

If two pipes are buried in a trench it is likely that they are

bonded together with a metal strap somewhere, whichwill thus give a path for electrons and cause galvaniccorrosion.


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3- Insulation: Non-metallic couplings, unions and flange insulation are

widely used for insulating against bimetallic corrosion.

However, they are not always wisely used. Allunderground pipes must be insulated from the above-ground pipes.

Insulators must be installed on distribution mains whenconnecting new steel pipes to old steel pipes, whenconnecting steel to cast iron and when installing a newlycoated pipeline at every 2000 ft.

When connecting a copper service pipe to a steel main,insulation is needed where the copper pipe connects

with steel main pipe. No insulation is necessary when joining a plastic pipe to

steel mains.


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ACCESSIBILITY FOR MAINTENANCE / PAINTING

The design must be able to allow easy access to the areas requiringrepair or maintenance.

Appropriate long-life paints should be applied in areas which maynot be accessible for a sufficient length of time.

Figure 8.37shows access to areas suitable and unsuitable forpainting.

A good design should allow uniform painting to be applied on thesurface. Areas of uneven coating thickness are potential sites forinitiation of corrosion.

Figures 8.38a and b illustrate the point. For uniform coatingapplication grind all sharp edges and apply an extra coat of paint.Keep sharp edges to a minimum.


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DESIGNS FOR LIQUID CONTAINERS

A good design for liquid container must offer the following:

a) Freedom from sharp corners and edges.

b) Smooth flow of liquid from the container.

c) Freedom from the buildup of water traps around the corners.

d) Complete drainage from the corners without any water traps.

The elimination of water traps is essential to minimize theformation of differential oxygen cells which lead to corrosion.

As an operational matter, it is essential to remove water anddry out stainless steel tubing without delay as soon as leaktesting of new water treatment plant is completed; there are

many examples of microbial corrosion causing severe pitting ofnew plant soon after leak testing.

e) Minimizing of bimetallic corrosion by joining compatiblematerials without the risk of galvanic corrosion.


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f) Complete internal and external coating of the containers, if costeffective.

Some of the above measures to prevent corrosion in

liquid containers are shown in Fig. 8.39.

Figure 8.39(a) shows the best design because of thecapability of the liquid containers for complete drainageand absence of water traps.

Figures 8.39 (b) and (c) are examples of bad designbecause of the incapability for complete drainage andpresence of water and moisture traps around thecorners.

Better designs are shown in Figures 8.39(d) and (e).

Figure 8.40(a) shows a bad design because of the

joining of a copper pipe with the galvanized steel tank.


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The copper ions may be plated on the surface of

galvanized steel and lead to pitting.

An aluminum inlet pipe joined to an aluminum tank would

not cause galvanic corrosion (Fig. 8.40b).

The design also offers a good drainage of the liquid. Thedesign could be further improved by further smoothing

the corners.


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A: bad design

B: good design


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SELECTION

OF

MATERIALS

MATERIALS EVALUATION AND SELECTION


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MATERIALS EVALUATION AND SELECTION

Material selection is critical to engineering design.

Corrosion may be minimized by employing anappropriate design (as discussed earlier).

The selection of appropriate materials in a givenenvironment is a key factor for corrosion control strategy.

The material selected (for specific service conditions)has to meet the criteria for:

mechanical strength

corrosion resistance, and

erosion resistance


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FACTORS AFFECTING

PERFORMANCE OF MATERIALS

The following is a list of factors affecting the performance of materials:

1) Expected performance and functions of the product.

2) Physical characteristics.

3) Strength and mechanical characteristics.

4) Corrosion and wear characteristics.

5) Fabrication parameters.

6) Recycling possibilities.

CORROSION AND WEAR CHARACTERISTICS


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CORROSION AND WEAR CHARACTERISTICS

Improper selection of materials without consideration oftheir corrosion behaviour in aggressive service

environment can lead to premature failureofcomponents and plant shutdown.

To avoid failures caused by corrosion:

Materials selected should be compatible with theenvironment.

They must possess sufficient resistance to corrosionfor the designed life.

Appropriate preventive maintenance practice must beadopted to maintain the integrity of theequipment/component.


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The selection of materials must be based on anextensive knowledge of the service environment.

The behaviour of materials largely depends upon thefollowing:

(a) Corrosive medium parameters (Figure 9.1).

(b) Design parameters

(stresses, bimetallic contacts, crevices, joints, sharpcorners, non-homogeneous surface, etc.).

(c) Materials parameters (NEXT).


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Materials Parameters


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Materials Parameters

The following are the material parameters which may affectcorrosion resistance:

1) Impurity segregation on grain boundaries.

This leads to weakening of grain boundaries andaccelerates corrosion attack.

Examples:

In 18-8 steels, depletion of chromium due to formationof chromium carbide promotes inter-granular attack

(IGA).The formation of Mg2Si at the grain boundaries in Al-Mg

alloys leads to weakening of grain boundaries andpromotes corrosion of Al-Mg alloys.


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2) Micro-structural constituents.

A heterogeneous microstructure forms anodic and

cathodic sites which promotes corrosion. Intermetallicprecipitate serves as anodic and cathodic sites and theymay be anodic or cathodic to the matrix.

Examples:

CuA12 precipitate is cathodic to the aluminum matrixand Mg2Si is anodic to the matrix.

Al 6013-SiC composite corrodes at the Al matrix / SiCinterface because of the preponderance ofintermetallic secondary phases.

A non-homogeneous distribution of SiC contributes toaccelerated corrosion.

3) Surface treatment such as galvanizing


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3) Surface treatment, such as galvanizing,phosphating and painting increase the resistance ofmaterials to corrosion.

Examples:

Galvanizing improves the resistance of steels tocorrosion and is widely used in automobile industry.

Galvanized pipes are widely used for transport of hotwater in domestic plumbing systems.

If the temperature, however, exceeds 650C, reversalof polarity (steel becomes anodic) can cause

corrosion of the galvanized pipes.


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4) Alloying elements and film formation.

Examples:

Alloying elements in steels, e.g. Cr, Ni or Mocontribute to the production of a protective oxide layer

which makes steel passive.

Addition of Cu to Al alloy increases the strength but

decreases the corrosion resistance.Scandium addition to Al-Mg-Si alloys significantly

increases their strength without effecting a decrease

in their corrosion resistance.

Coatings of epoxy and polyurethanes provide alonger life to structures exposed to corrosive

environments.

FABRICATION PARAMETERS


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FABRICATION PARAMETERS

Following are the fabrication parameters required foranalyzing material selection:

1) Weldability.

Welding procedures, such as electric arc weldingfriction welding, spot welding, need to be carefullyselected to minimize the effect of corrosion.

For instance, gas welding in the sensitivetemperature

range may cause intergranular cracking.

2) Machinability. Machining operations, such as drilling, milling,

shearing, turning, may lead to enhancement ofcorrosion if they are not properly controlled.

Drilling fluids are highly corrosive and need to be

handled with care.


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3) Surface modification procedures (e.g. cladding i.e.covering, galvanizing and metallizing applying metalliccoatings):

They increase the resistance of the materials tocorrosion.

The success of the coating depends on the bondingbetween the coating and the substrate and surfacepreparation before the application of coatings accordingto the international standards.

Surface preparation techniques include hand, power toolcleaning and abrasive blast cleaning.

STRATEGY OF MATERIALS SELECTION


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STRATEGY OF MATERIALS SELECTION

The following steps lead to the screening and ranking ofcandidate materials:

a) The material attributes must meet the required designrequirements.

For example, if the service temperature for a commercial

steel component in an oxidizing environment isprescribed at 1100C, 25 Cr-20 Ni steel would be asuitable choice.

Other steels, such as 13 Cr Steel, 18 Cr steel would bescreened out because their limiting service temperatureare below 7500C.

b) Find candidate materials which can do the job. Howgood a job can be done by other materials can be knownby determining their material indices.

A bi ti f h i l ti hi h


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A combination of mechanical properties which

characterize the performance of material gives

maximized or minimized material attributes.

c) Supporting information: Screening helps to

short list the candidate materials and considerably

narrows down the range of selection.To further narrow the choice of candidates,

support information is explored comprising:

1) Handbooks, such as ASM Handbook, CRC Handbookof Materials, etc.

2) Data sheets from suppliers.


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3) Data sheet from professional organizations, such as

Nickel Development Association (NDA); Copper

Development Association (CDA); American Iron andSteel Institute (AISI), etc.

4) Websites of suppliers.

5) Personal contacts.

d) Local environment:The suitability of a product or

equipment in a local environment is essential to service

life.

e) The final material choicewill evolve the completion of

a systematic screening process.

MATERIALS AND FLUID CORROSIVITY


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MATERIALS AND FLUID CORROSIVITY

Corrosion resistance of materials depends on the natureof the corrosive environment.

A material may be very corrosion-resistant in anenvironment and yet fail in another environment.

A knowledge of corrosion resistance of a material in a

given environment is, therefore, of a fundamentalimportance to their successful application. Aknowledge of the corrosivity of the environment, istherefore, very useful for selection of materials.

Water plays a predominant role in a corrosionreaction since it is an electrolyte, an essentialcomponent of a corrosion cell.

Pure water is a poor conductor arid not significantly

corrosive up to 170C.


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In the presence of salts, acids and dissolvedgases, like hydrogen sulfide, carbon dioxide

(CO2) and oxygen (O2), the degree of corrosivityof water is significantly increased.

The following are the common aggressive

environments encountered by materials:

1) Industrial environment

2) Marine environment.

3) Oilfield environment.4) Pollution environment.

INDUSTRIAL ENVIRONMENT


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INDUSTRIAL ENVIRONMENT

The industries contributing to the industrial environment

include chemical, petrochemical, fertilizer, pulp and paper,and all other process industries.

The following are the major corrosive fluids encountered ina wide spectrum of industry:

a) Acids (inorganic & organic, i.e. mineral & carboxylic).

b) Strong alkalies (bases).

c) Salt water.

d) Dissolved gases, e.g H2S, CO2, and oxygen.


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e) Pollutants, e.g. particulate matter, sulfur oxides, CO,

NOx, ozone and lead.

f) Soil pollution, e.g. bacteria, oil spills, natural gas

contaminants, sewage contaminants and pesticide

degradation products.

The success of service performance of materials

would depend on their ability to offer sufficient

resistance to corrosion in industrial

environments.


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RANKING OF PERFORMANCE OF MATEREALS

The ranking of materials is based on the basis of the degree of

corrosion resistance they offer in a given environment. The best way to evaluate corrosion resistance is to determine their

rate of corrosion according to the standards of:

ASTM (American Society for Testing of Materials)

NACE (National Association of Corrosion Engineers)

COST EFFECTIVENESS IN MATERIAL SELECTION


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COST EFFECTIVENESS IN MATERIAL SELECTION

A. Design life

This is based on a specified amount of time required to

recover investment and operating costs, plus areasonable profit margin.

B. Quality level

A quality level assigned to an item of fabricatedequipment indicates the required level of testing andinspection.

Quality can be defined as fitness for the purpose.

C. Corrosion allowance

Corrosion allowance is related to the corrosion rate andthe designed life of the equipment.

Carbon steel is cost effective because it remains


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Carbon steel is cost-effective because it remainseconomical even after the adding cost of extra wallthickness required to maintain corrosion allowance.

D. National codes

National codes and standards can be used as guidelinesfor selection of material and fabrication of equipment.

E. Economic consideration

Initial cost, operating costs and maintenance costs mustbe given full consideration in the process of materialsselection.

F. Availability

This is an important consideration. An alternate materialmust also be specified in case the recommended

9 corrosion prevention design material select

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