Page 37 - Valve Selection Handbook

24 Valve Selection Handbook

Corrosion of stainless steel valve stems by packings. Stainless steel
valve stems—in particular those made of AISI type 410 (13Cr) steel—
corrode frequently where the face contacts the packing. The corrosion
occurs usually during storage preceding service, when the packing is sat-
urated with water from the hydrostatic test.
If the valve is placed into service immediately after the hydrostatic
14
test, no corrosion occurs. H. J. Reynolds, Jr. has published the results of
his investigations into this corrosion phenomenon; the following is an
abstract. 15 Corrosion of stainless steel valve stems underlying wet pack-
ing is theorized to be the result of the deaerated environment imposed on
the steel surface by the restricting packing—an environment that influ-
ences the active-passive nature of the metal. Numerous small anodes are
created at oxygen-deficient sensitive points of the protective oxide sur-
face film on the stainless steel. These, along with large masses of
retained passive metal acting as cathodes, result in galvanic cell action
within the metal. Graphite, often contained in the packing, acts as a
cathodic material to the active anodic sites on the steel, and appreciably
aggravates the attack at the initial corrosion sites through increased gal-
vanic current density.
Because of the corrosion mechanism involved, it is impractical to
make an effective non-corrosive packing using so-called non-corrosive
ingredients. Incorporating a corrosion inhibitor into the packing is thus
required, which will influence the anodic or cathodic reactions to pro-
duce a minimum corrosion rate. Of the anodic inhibitors evaluated, only
those containing an oxidizing anion, such as sodium nitrite, are efficient.
Cathodic protection by sacrificial metals such as zinc, contained in the
packing, also provides good corrosion control. Better protection with a
minimum effect on compression and serviceability characteristics of the
packing is provided by homogeneously dispersed sodium nitrite and a
zinc-dust interlayer incorporated into the material.
High chromium-content stainless steels—especially those containing
nickel—exhibit a marked increase in resistance to corrosion by inhibited
packing, presumably because of the more rapidly protective oxide sur-
face film and better retention of the passivating film.

Lip-Type Packings

Lip-type packings expand laterally because of the flexibility of their
lips, which are forced against the restraining side walls by the fluid pres-
sure. This mode of expansion of the packing permits the use of relatively