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Protection From Corrosion
By Mahmood NaqviKarachi – Pakistan
Protection From Corrosion What causes rusting?
A process started by the interaction of air, moisture & metal
How do we prevent it?By putting on the metal a barrierImpermeable to air and moisture
Chief Principles for Prevention Barrier forming paints
Conventional paints, epoxy, vinyl paints Active rust preventing zinc rich paints
Red led paints,zinc chlorate primer Problems with zinc rich paints
Lead compounds make the paint poisonous
Linseed oil impractical because of long drying time
Surface Preparation Critical because different surfaces have
different requirements Sharp surface projections should be
ground smooth. Grinding should not effect thickness
Boreholes should be, drilled & smoothed Oil, grease & other contaminations should
be removed
Pretreatment Importance of Pretreatment
The Swedish standard sis 055900 is used to ensure quality
Effects the results obtained Effects the costs involved
Different Corrosion GradesA. Steel surface with little if any rust,
completely covered with mill scaleB. Steel surface has begun to corrode,
mill scale has begun to flakeC. Steel surface with visible pitting, mill
scale corroded awayD. Steel surface with considerable pitting,
mill scale has corroded away
Pretreatment For Corrosion Grades Pretreatment through wire brushing &
blast cleaning Bad (light brushing & scraping) Middle quality ( scraping with scraper,
wire brushing & blast cleaning) Scraping, wire brushing or blast
cleaning till clean grey steel
SIS 055900 Norms: ST: Wire brushing Sa: Blast cleaning ST1, ST2, ST3 & Sa1, Sa2, Sa3 Norm between Sa2 & Sa3 found ideal As a result Sa 2 ½ was made
Method & Tool Selection Depends on:
Degree of damage to the surface Which paint system is to be used The nature of the surface Previous treatment Results required
The Ideal Maintenance Job Removal of rust and mill scale Removal of grease,oil, dust Removal of unwanted paint Abrasion of sharp edges Malting of glossy paint Thorough removal of dust
Methods Of Paint Removal Method
Blast cleaning Mechanical wire
brushing Mechanical disc-
sanding Needle chipping Mechanical rust
chipping Hand brushing Hand scraping
Result Ideal Risk of polishing Risk of indentations Bad heavy indentations Good with other methods Poor Only use with other
methods
Classifications of Surface Preparation:
Solvent cleaning (SSPC-SP-1) Hand tool cleaning (SSPC-SP-2 st2) Power tool cleaning (SSPC-3, SIS st3) Brush-off blast (SIS Sa-1, SSPC-SP-7, NACE-4) Commercial blast (SIS sa2, SSPC-SP6, NACE-
3) Near white metal blast (SIS Sa-2.5,SSPC sp-10,
NACE-2) White metal blast (SIS Sa-3, SSPC SP-5, NACE-
1)
Rules for Pretreatment Use correct pressure 7-7.5 kg/sq.Cm (100-107 p.S.I ) Loose paint & rust must be removed Use the correct properties of sand Always use good quality sharp quartz sand Remove dust from the blasted surface Small area at a time Protect with primer Paint with selected paint system
Blasting Equipment Selection Compressors: ideal working pressure 90-100. Nozzle: should be matched with discharge
volume capacity.Compressor rated at 125cfm at 100 psi. One orif ice of ¼ inch dia, delivered 81 cfm. Two orif ice of 3/16 inch dia delivering 81 cfm. One orif ice of ½ inch dia plus one orif ice of ¼
inch dia.Compressor rated at 600 cfm at 100 psi. Both delivering a total of 419 cfm. One orif ice of 7/16 inch diameter. Plus one
orif ice of 3/8 inch diameter. Both delivering a total of 450 cfm..
Continued… Abrasive: quality of surface affected by
size and shape of abrasive used.Preferred abrasives.
16-20 mesh silica sand or mineral grit. 20-40 mesh garnet deep red color. Crush iron slag. G-50 irons grit.Have a surface profile 1.5-3.5.
Abrasives continued…
Abrasive Size Mils Microns
Large River Sand
US 12 – US 50 2.8 70
Medium Silica Sand
US 18 – US 40 2.5 62
Silica sand US 30 – US 80 2.0 50
Continued.. Miscellaneous
Use dry abrasive, free from dust, salt and other impurities
Keep moisture, organic matter of surface Apply coating as soon as possible to
prevent surface rusting Dry cleaning should not be done in rain,
mist, fog or humidity levels above 85%
Coating Material
General guidelines: Coating material should be protected from rain dust & sunlight. Should be inspected when opened for use. Surface should be rendered dust free prior to coating. Should be mixed thoroughly for sufficient time as specified by
manufacturer. Pot life of catalyzed coating should not be exceeded. No coating when surfaces are less than 5 degree above dew point. Manufacturers recommendations regarding curbing time should be
followed.
Coating Material Specifications Depends on service requirements
Vary from general atmospheric exposure to salt water immersion
Depends on the degree of surface preparation
Materials must be checked for health, safety and environmental standards
Coating Application Spray Equipment
Lines & pots must be cleaned Must be an adequate moisture trap Suitable working regulator & gauges for air supply to
pressure pot & pressure gun Spray equipment must be grounded For airless spray 28:1 or 30:1 pump ratio should be
used. Air supply should be 80-100 psi For conventional air supply equipment should include
pressure pot with mechanical agitator, separator atomizing and fluid pressure regulators
A supply of tips with varying spray angles and washers
Coating Application
Spray application Gun should be at right angles to surface For airless spray the spray gun should be held between
1 ½ to 2 feet For air supply the spray gun should be held between 6
to 10 feet Even and parallel passes should be made with spray
gun, each spray should overlap the other by 50% Coating should be applied uniformly and completely
over the surface Areas such as corners & welds should be precoated by
brush to ensure they get at leas the minimum film layer
Coating Application Brush application
Brushes should be of suitable size and quality Brushing should result in a smooth and uniformly thick
coat, there should be no brush marks Runs or sags should be brushed out Successive coats are applied by cross hatching the
previous coat Care must be taken that no lifting of former coats
occurs Areas such as corners & edges should receive extra
coating to ensure the minimum level is met
Inspection Good house cleaning
Clean and orderly work area Storage and issue area Mixing utensils and containers Wet coating safe grades
Surface preparation Prior to surface cleaning
Careful examination Recording of inspector observations Repair of damaged areas Surface must be free from hydrocarbon and water
Inspection
Surface Preparation Blast cleaning equipment
Correct nozzle size Minimum 2 inch internal diameter hose and
coupling Blast hose of sufficient size to maintain nozzle
pressure All couplings should be external type Remote control valves fitted for safety ?????????????????????
Inspection
Surface preparation Abrasive medium: grit/ sand should be inspected to
ensure it is clean Final inspection
All phases of surface preparation must be completed before final inspection
A time lapse of at most 2 hrs is allowable between completion of blast cleaning and final inspection
Cleaning clothe must not be used to inspect surface The inspector must must monitor the condition of freshly
blasted or cleaned areas Degree of cleanliness can be confirmed based on surface
texture, shade or by using the Swedish standard
Inspection Coating application
Application procedure Preliminary inspection Homogenous mixing Thinners Site control of material
Equipment Airless spray equipment Brush application
Inspection
Coating inspection Quality control measures
Wet film thickness Dry film thickness Holiday detection
Safety General: a safety meeting must take place prior to
job Good house keeping: unnecessary people and
equipment pose obstacles to workers Sand blasting:
Remote control valve for safety Dust evacuators and canvas screens for work area Properly trained certified personnel Unnecessary people kept away Adequate light Secure and adequate scaffolding
Safety Coating application
Adequate equipment grounds Non-conductive hoses Any other protective devices Keep clear from spray stream Handle spray gun carefully Never point at anyone Do not cover the fluid tip Use necessary safety gear for personnel
equipment Never exceed the recommended inlet pressure
Safety Coating application continued…
Know the working pressure of accessories used with the gun Never exceed 5000 psi working pressure Before removing equipment shut down the pump and
decrease system pressure Never to clean the tip when installed in the gun Never operate airless spray equipment until all static
electricity discharge grounds are in place and connected Never flush solvents into containers exposed to direct
sunlight When a tank is out of service or unattended all accesses
must be closed so that coating may dry up