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Materials of Construction and Surface Finishes 205
removes iron without removing significant amounts of nickel and chromium thus
producing ahigher Cr/Fe ratio than mineral acids.
Because of citric acid’s high reactivity with free iron and low reactivity with
other metals, passivation is done at lower temperatures and shorter residence time.
Typical passivation conditions might be 1508 F(658 C) for 30 minutes using 10% citric
acid. Citric acid is nonhazardous, does not produce nitrogen oxides, does not
require special handling equipment or safety devices, and does not corrode other
equipment, finishes, and structures.
Chelants
Iron can be removed from stainless surfaces by citric acid alone but inclusions
require additional treatment. Chelants are added to enhance free iron removal and
help remove other impurities such as sulfides, calcium, aluminum, manganese,
silica, carbon, and silt. Five or more ingredients (surfactants, acid chelants, buffer
agents, and stronger reducing acids) may be added to aid in dissolving and
removing contaminants.
Welding and the HAZ
General
Welding produces heat tint discoloration as alpha ferrite is converted to delta
ferrite. The resulting magnetism can lead to galvanic action and then corrosion.
The welding process changes the surface chemistry from the weld bead through
the heat affected zone (HAZ) and may dramatically reduce the corrosion resistance
throughout the weld area because the Cr/Fe ratio is lowered and manganese
concentrates in both the weld area and the HAZ.
Conventional passivation will not adequately protect the HAZ because the
depth of the HAZ tinting and disruption is greater than can be addressed by
passivation alone. Although the surface layer will be passive, the surface just below
the chromium oxide is ripe for attack if the passive layer is breached.
Resolution of heat tint problems requires avoiding the formation of heat
tint. This is done using orbital welding procedures with appropriate high-purity
gas purge and avoiding the welding of dissimilar materials. Poor quality welds
should be rejected, cut out, and replaced by acceptable welds to prevent future
problems.
Dissimilar Materials
Gaskets, valve packing, diaphragms, and hoses must be carefully specified, stored,
and installed to avoid introducing iron or other foreign material into a
process system.
Dissimilar metals particularly at welds may cause an attack on the structure
and integrity of stainless systems by forming corrosive galvanic cells.
Machining and Heat-Treating Techniques
Contamination introduced during manufacturing or thermal processes may lead to
corrosion. Manufacturing processes should be reviewed to minimize the possibility
of cross-contamination during manufacturing and increase the chances of
successful passivation and tests results.