Module 3 “Engineering Considerations”

• Materials Selection and Testing

• Equipment Design

• Inspection for Corrosion- Process Check Points

• Formulating to Avoid Corrosion Tendencies

• Engineering Materials- Critical Points

• Database Resources

• Review

Materials Selection Process

• Depends on Operating Conditions

• Review of Process Components

• Consideration of Materials

• Evaluation of Materials

• Follow-Up Monitoring

Processes and packages are made out of many different types of materials of construction

Depicted here pumps, valves and tanks to show variables that add complexity

All at different corrosion potentials

Raw Materials

Final Products

Operating Conditions

• The chemical environment should be defined precisely, including the presence of trace compounds.

• Temperatures, pressures, flow rates, liquid vs. gas, aqueous vs. non-aqueous, continuous vs. intermittent operation, media used for heating or cooling, and product purity requirements.

• Abnormal or upset conditions.

• pH range and salt levels

Review Process Components

• Size

• Complexity

• Criticality in service

- For example: Consider critical , unique pieces of equipment in a single train continuous process, where one component failure would shutdown the entire operation. Rotary valve- Oral Care, Soap Column-Continuous Soap making.

Selection of Materials of Construction

• Pertinent past experience

• Making a list of materials to choose from and their properties

• Vendors ( we rely to heavily on them)

• Literature survey

• MEIR database

Testing of Materials

Corrosion testing is needed when:

1. There is uncertainty about the right material to use

2. Significant consequences if the wrong material of construction is used.

- Consequences- Product contamination or recall, liability, safety.

Objectives of Laboratory Corrosion Testing

• Predict service life

• Determine the suitability of a engineering material for a given application

• To rank new engineering materials

General Corrosion Testing Guideline

• Use a testing procedure that fits the situation, consult if necessary.

• Observe samples carefully before and after testing

• Take pictures before and after for comparison

• Simulate the service environment as closely as possible

• Effects to take into account; trace impurities, suspended solids, aeration, temperature and pH.

• Corrosion samples can be ordered from Metal Samples in Munford, Alabama http://www.alspi.com/ms.htm

Corrosion Testing Example

U Bend testing to determine the relative stress corrosion cracking resistance of 304L, 316L and 2205 alloys

Lab Data pH and Chloride effects on 304 SS

• pH 2 pitting at 0.1 % CL at 25 C, cracking at 60 C

• pH 7 pitting at 0.1 % CL at 60 C, cracking at 80 C

• pH 12 no pitting at B.P., cracking at 1% CL at B.P.

• Test duration approximately 1 year

Handout on Material Selection

• See section on “Materials Selection” which has an in depth discussion on this subject

• Pages 321-325

Equipment Design

• Use proper welding techniques - problems with

stainless, eg., heat affected zone (HAZ); therefore use stabilized grades “L” or Titanium stabilized in Europe.

• Select the right weld filler materials for the project

• Use of dissimilar metals should be avoided

• “Mechanical” design at lower stresses

• Underground structures need corrosion protection

General

Enough distance for coating

Not DesirableDESIRABLE

Weld not accessible for grinding

Not enough distance for coating

No radiusWeld accessible

Radius

Design Considerations

Steel

Pit (anode)

Alloy (cathode)

Protective coating

Joining Dissimilar Metals

Coating holiday-Coating holiday-undesirableundesirable

Skip weld

Welding ConsiderationsSKIP WELDS

Crevice-Crevice-undesirableundesirable

Special attention requiredDesirable

Continuous weld

Skip weld

Welding Considerations

Flush

RoughSmooth

Smooth contour (as-welded or ground) for

coating

Undercut

Rollover

Porosity

Welding Considerations

Good Good UndesirableUndesirable

Sharp corner

Inside of vessel

Corrosion source Threaded parts

Desirable Flange outlets

Weld

Inside of vessel

Round corners

2”

Min.

Higher Energy state

Grind smooth

UndesirableDesirable

Continuous fillet weld

Gap

Weld

Inside of vessel

Gap

Crevice

Crevice

Crevices

AVOID AVOID BETTER PREFERRED

Joining Details

Crevices and Drainage

• Flanges, threaded connections, drain lines should be avoided. Invitation to pitting by oxygen concentration cells.

• Heat exchanger crevices are highly susceptible to attack

• Low legs in piping are to be avoided as they may retain water

Heat Exchanger Tubing

• Seamless tube- What to watch out for-

Extrusion tears, Lap defects (stringers)

• Welded tube- Pinholes: test at 150 % of the highest working pressure

Velocity Corrosion Control

• Piping high end velocity 1.2 to 1.8 m/sec (4-6 ft/sec)

• Tubing/heat exchangers up to 8 m/sec (25ft/sec)

• Orifice plates, throttling valves, pump impellers – erosion resistant materials are needed

Stress Relief

• Two types

- Thermal

- Mechanical

Thermal

• Through heat treatment of stainless steel we can lower residual stresses and therefore the risk to stress corrosion cracking.

• Typical temperatures are above 1000 F or 600 C ( Heat Treat Furnace )

• Possible application would be welded mixer blades.

Mechanical

• Consists of shot peening the surface of an item or equipment with an appropriate type of shot

• Surface in compression is more resistant to mechanical fatigue, corrosion fatigue and environmental cracking.

• Applications – shafts and blades etc.

Corrosion Allowance

• Use for general corrosion situations, i.e.. Mainly carbon steel

• The idea is to leave the required thickness for piping or vessels that will meet the mechanical design requirements of temperature and pressure over the estimated useful life.

• This approach is practical for allowances of 3 to 6 mm or (0.13 to 0.25 in. )

Corrosion Inspection

Survey Plant sites for materials of construction issues

• Why

- To avoid possible product quality issues

- To avoid plant process failures ..

Process failures lead to safety, environmental and liability

issues

Key Operational Checkpoints

• Elevated temperatures above 140F ( 60 C)

• Raw Materials (acids and base)

• Anywhere your using salt (NaCl)

• Anywhere your using abrasives

• High flow rates ( > 1.2 M/sec velocity)

Key equipment Checkpoints

• Tanks, Piping , Mixers, Heat Exchangers, Valves

• Moving Parts such as in Fillers

• Crevices

• Heterogeneity- more than one material of construction.

• Equipment operating under pressure or vacuum

Other Critical Inspection Points

• On-Site water piping

• Steam and Utility piping

• Underground lines or piping

Plant Inspection Tools

• Visual Observation

• Ultrasonics- Tanks- General corrosion

• Magnetic Particle- Welds and stress corrosion

• Depth Gauge- localized corrosion, pitting

• X-rays – Tank and equipment replacement decisions

• Boroscope- Optical fiber microscope for determining corrosion severity in hard to reach places such as tubes in a heat exchanger

See Handout on Inspection and Failure Analysis

• Chapter 38 Inspection and Failure Analysis

• Inspection techniques see pp. 290-292

Ultrasonic Thickness Testing

Determining the wall thickness of storage tanks tells us the margin of safety and the degree of risk from an environmental spill.

Close-Up Inspection

• Of plant machinery and processing equipment is a way of determining if any corrosion is occurring.

• This is especially important on stainless steel parts.

• Early signs pitting or cracking could be a serious business issue.

Manufacturing Corrosion ExerciseYour given the added responsibility of making sure your plant does not incur any significant corrosion issues that could impact manufacturing downtime, EOHS, or product quality.

Givens:

Your plant uses NaCl to make Softsoap products. Your plant also makes Toothpowder using 440 stainless rotary valves, Soap/Lyes, Glycerin, Detergents Liquids and Powder.

Based on Module 1, Module 2, and what you learned so far in Module 3, What areas in the plant would you check for corrosion and how?

Hints- Use the examples given, the inspection checklist and tools, and the section handout on “Inspection and Failure Analysis.”

Formulating to Avoid Corrosion Tendencies

Salt ConcentrationNaCl

• Sodium chloride is the worst possible component from a corrosion standpoint in a formula.

• Minimizing it is important, less than 1% would help to prolong stainless steel equipment life and help minimize micro issues.

pH (Acidity/Alkalinity) values

• pH extremes are bad for engineering materials.

• Corrosion potential goes up significantly below a pH of 5.5 and above 9.

• There are of course exceptions to this guideline, for example under erosion conditions high pHs are generally better.

Formula Corrosion Inhibitors

• Sodium nitrate ( for Cl- ) pitting

• Sodium silicate ( general corrosion)

• Sodium sulfate ( for Cl-) pitting

• Concentration and type to use depends on the circumstances.

Engineering Materials- Critical Points

Metal Behavior• Different metals corrode differently due to there metallurgy

- Metallurgy: Science of making metals from raw materials

• Some effects

- Carbon Steel (general corrosion)

- Gray cast iron (graphitization)

- Ductile iron (usually pitting, some graphitization)

• Stainless steel

- Passivity due to oxide film

- Susceptible to corrosion in various environments

- Need to select the proper stainless for the environment

Types of Stainless Steels• Martensitic

- 12% – 17% Cr

- minor other elements

- moderate corrosion resistance in mild environments, heat treatable

• Ferritic

- higher Cr composition than Martensitic (12% – 30%)

- better corrosion resistance to high temperatures

Stainless Steels (cont)• Austenitic

- Cr: 17% – 25%

- Ni: 9% – 10%

- generally more corrosion resistant than martensitic (304/316)

• Duplex alloys (2205/2507)

- mixture of ferritic and austenitic

- increased strength and corrosion resistance, for stress cracking environments

Non Metallic Issues• Concrete

- Freeze thaw cycle- leads to cracking

- Rebar corrosion

- Attack by de-icing salts and acids

- Hydrogen sulfide attack in sewers

• Plastics

- UV attack

- Thermal stresses and cracking

- Solvent attack Resins- FRP

Fiberglass Material

GlasGlasss

ResiResinn

NANA

FRP Attack Delamination

Resin Resin dissolvdissolv

eded

Composite (FRP) Tank FailureProblem: Internal PVC

liner weld failed mechanically.

Bleach then attacked inner FRP resin (non-compatible) glass support structure.

Could not see the damage to the outer liner.

Solution : Redesign tanks without liners.

FRP Impact Cracks

Coatings and Linings

• Separate material from the environment

• Provide good adhesion to the metal

• Serve as primer for topcoats

• Provide sacrificial protection to the metal

• Resist abrasion, impact and soil stress

• Hold an inhibitor at the metal surface

• Resist water absorption

Functions and qualities of coatings

Functions and Qualities of Coatings (cont)

• Prevent contamination

• Protect against high temperature oxidation

• Be safe to use

• Be environmentally acceptable

• Provide good electrical insulation for underground structures

Coating Application Considerations

• Surface preparation

• Atmospheric conditions

• Application techniques

• Inspection

- Wet and dry film thickness

- Electrical (holiday) inspection

Surface Preparation-Blasting

Coating Testing for Continuity

Checking for pinholes in the coating that would be sites for corrosion to occur

MEIR Database

MEIR’s• Materials of Engineering Information

Requests

• Historical information

• Information also in hard copies and index.

• Many chemicals are included

Resources

• NACE International Corrosion Society- www.nace.org

• ASM American Society of Metals- www.asminternational.org

• ASTM American Society of Testing Materials- www.astm.org

• ASME American Society of Mechanical Engineers- www.asme.org

NACE NACE InternationalInternational

InhibitorsInhibitors

Cathodic protectionCathodic protection Coatings and Coatings and LiningsLinings

Materials Materials Selection and Selection and

DesignDesign

NACE Technical CommitteesNACE Technical Committees

NACE “International Corrosion Society”NACE “International Corrosion Society”

WaterWater

What did we learn?• A guideline for selecting materials of construction

• There are different kinds of stainless alloys and the selection of the right one for the specific application is important

• Metallic and non-metallic materials of construction have there Achilles heels.

• Design with lowest stresses possible, eliminate corners, provide smooth welds, and finish

• Key materials of selection points; high temperature, areas of wear, highly stress parts.

• There is historical data on engineering materials ( MEIR’s)

• Resources for corrosion information – such as NACE and ASM

• A checklist for conducting Corrosion Audits.

—RememberRemember—

Corrosion Corrosion Engineering Engineering

Doesn’t Cost — Doesn’t Cost —

It Pays!It Pays!