Corrosion of Linings
179
Vinylidene chloride latex:
•
excellent fuel oil resistance.
Alkyds, epoxy esters, oleoresinous primers:
•
water immersion applications and as primers for other topcoats.
Inorganic zinc, water-based postcure, and water-based self-cure:
•
Jet fuel
storage tanks and petroleum products.
Inorganic zinc, solvent-based self-curing:
•
excellent resistance to most
organic solvents (aromatics, ketones, and hydrocarbons); excellent
water resistance; difficult to clean; may be sensitive to decomposition products of materials stored in tanks.
Furan:
•
most acid-resistant organic polymer; used for stack linings
and chemical treatment tanks.
Answers to the next set of questions will narrow the selection to those
materials that are compatible, as well as to those coating systems that have
the required physical and/or mechanical properties:
1. What is the normal operation temperature and temperature range?
2. What peak temperatures can be reached during shutdown, startup,
process upset, etc.?
3. Will any mixing areas exist where exothermic heat of mixing may
develop?
4. What is the normal operating pressure?
5. What vacuum conditions and range are possible during operation,
startup, shutdown, and upset conditions?
The size of the vessel must also be considered in coating selection. If the
vessel is too large, it may not fit in a particular vendor’s oven for curing of
the coating. Also, nozzle diameters 4 in. and less are too small to sprayapply a liquid coating. When a coating is to be used for corrosion protection,
it is necessary to review the corrosion rate of the immersion environment
on the bare substrate. Assuming that the substrate is carbon steel with a
corrosion rate of less than 10 mil per year (mpy) at the operating temperature, pressure, and concentration of the corrodent, then a thin film lining
of less than 20 mil can be used. For general corrosion, this corrosion rate is
not considered severe. However, if a pinhole should be present through the
lining, a concentration of the corrosion current density occurs as a result
of the large ratio of cathode to anode area. The pitting corrosion rate will
rapidly increase above the 20-mpy rate and through-wall penetration will
occur in months.
When the substrate exhibits a corrosion rate in excess of 10 to 20 mpy, a
thick film coating exceeding 20 mil thickness is used. These thicknesses are
less susceptible to pinholes.
179
Vinylidene chloride latex:
•
excellent fuel oil resistance.
Alkyds, epoxy esters, oleoresinous primers:
•
water immersion applications and as primers for other topcoats.
Inorganic zinc, water-based postcure, and water-based self-cure:
•
Jet fuel
storage tanks and petroleum products.
Inorganic zinc, solvent-based self-curing:
•
excellent resistance to most
organic solvents (aromatics, ketones, and hydrocarbons); excellent
water resistance; difficult to clean; may be sensitive to decomposition products of materials stored in tanks.
Furan:
•
most acid-resistant organic polymer; used for stack linings
and chemical treatment tanks.
Answers to the next set of questions will narrow the selection to those
materials that are compatible, as well as to those coating systems that have
the required physical and/or mechanical properties:
1. What is the normal operation temperature and temperature range?
2. What peak temperatures can be reached during shutdown, startup,
process upset, etc.?
3. Will any mixing areas exist where exothermic heat of mixing may
develop?
4. What is the normal operating pressure?
5. What vacuum conditions and range are possible during operation,
startup, shutdown, and upset conditions?
The size of the vessel must also be considered in coating selection. If the
vessel is too large, it may not fit in a particular vendor’s oven for curing of
the coating. Also, nozzle diameters 4 in. and less are too small to sprayapply a liquid coating. When a coating is to be used for corrosion protection,
it is necessary to review the corrosion rate of the immersion environment
on the bare substrate. Assuming that the substrate is carbon steel with a
corrosion rate of less than 10 mil per year (mpy) at the operating temperature, pressure, and concentration of the corrodent, then a thin film lining
of less than 20 mil can be used. For general corrosion, this corrosion rate is
not considered severe. However, if a pinhole should be present through the
lining, a concentration of the corrosion current density occurs as a result
of the large ratio of cathode to anode area. The pitting corrosion rate will
rapidly increase above the 20-mpy rate and through-wall penetration will
occur in months.
When the substrate exhibits a corrosion rate in excess of 10 to 20 mpy, a
thick film coating exceeding 20 mil thickness is used. These thicknesses are
less susceptible to pinholes.