CORROSIONRESISTANT PART 4
NICKEL ALLOYS
Nickel alloys provide levels of
corrosion resistance not possible
with other alloys. This is part four
of a four-part series about
corrosion-resistant nickel alloys.
Paul Crook*
Haynes International Inc.
Kokomo, Indiana
Nitric acid
Unlike the three halogen acids discussed last
month, nitric acid causes a high potential (oxidizing) cathodic reaction that readily induces passive films on high-chromium stainless steels. In
fact, the stainless steels are very resistant to nitric
acid and the nickel alloys are not needed unless
there are extenuating circumstances, such as the
additional presence of halogen acids or halide salts.
Some nickel alloys exhibit good resistance to
nitric acid, notably those Ni-Cr and Ni-Cr-Mo materials with high chromium contents. Nitric acid
iso-corrosion diagrams are shown for C-2000 and
*Member of ASM International
Corrosion-resistant nickel alloys are used in the structures of chemical processing plants such as this chlorine/EDC/VCM complex. Image courtesy Uhde,
a company of ThyssenKrupp Technologies.
Boiling point curve
Over 0.5 mm/year
0.1 to 0.5
mm/year
80
40
Under 0.1 mm/year
20
40
60
Acid concentration, wt%
Fig. 1 — Iso-corrosion diagram for
C-2000 alloy in nitric acid.
120
Temperature, °C
120
Temperature, °C
N
ickel is an ideal base for corrosionresistant alloys. Not only is it inherently resistant to certain chemicals, but
also it can be highly alloyed with elements known to enhance corrosion performance,
such as chromium, copper, and molybdenum,
while retaining its ductile face-centered cubic
structure. Iron is not as accommodating; thus high
levels of such elements are not possible in the
stainless steels without structural instability.
In addition to commercially pure nickel, three
binary alloy systems also provide exceptional corrosion resistance. These include nickel-chromium
(Ni-Cr), nickel-copper (Ni-Cu), and nickel-molybdenum (Ni-Mo). Chromium enhances the resistance of nickel to oxidizing acids by encouraging
the formation of passive films. Copper is very
helpful in seawater, brackish water, and reducing
acids, in particular hydrofluoric. Molybdenum is
extremely beneficial in all reducing acids.
This article focuses on the effects on various
nickel alloys of industrially important acids, bases,
and salts. These include nitric acid, phosphoric
acid, sulfuric acid, and caustic alkalis.
Boiling point curve
80
Under 0.1 mm/year
40
20
40
60
Acid concentration, wt%
Fig. 2 — Iso-corrosion diagram for
G-35 alloy in nitric acid.
G-35 alloys in Fig. 1 and 2. From these, the strong
influence of chromium content can be deduced.
The G-35 alloy, with a chromium content of 33
wt%, exhibits corrosion rates of less than 0.1
mm/y at all temperatures up to the boiling point
curve, at acid concentrations up to 70 wt%.
Phosphoric acid
About ten million tons of phosphoric acid are
produced annually in the United States, 80% of
which is used in the production of agricultural
fertilizers. Most of this acid is made by the “wet
process,” which involves a reaction between sulfuric acid and phosphate rock.
Iso-corrosion diagrams do not exist for fertilizer-grade phosphoric acid. This is partly because
impurity contents vary from country to country
ADVANCED MATERIALS & PROCESSES/SEPTEMBER 2007
45
160
120
0.3
0.2
0.1
38
40
42
44
46
48
50
Acid concentration, wt%
52
160
0.1 to 0.5 mm/year
Temperature, °C
Corrosion rate, mm/year
31 in acid 1
31 in acid 2
28 in acid 1
C-2000 in acid 1
80
Under 0.1 mm/year
40
20
40
60
Acid concentration, wt%
54
Fig. 3 — Corrosion rates in “wet process” phosphoric acid
(Source: Florida. Test temperature 121°C).
250
Boiling point curve
Temperature, °C
150
Over 0.5 mm/year
0.1 to 0.5 mm/year
150
Boiling point curve
0.1 to 0.5 mm/year
100
100
50
Temperature, °C
200
200
0.1 to 0.5 mm/year
Under 0.1 mm/year
0.1 to 0.5 mm/year
50
20
40
60
80
Acid concentration, wt%
Fig. 6 — Iso-corrosion diagram for
B-3 alloy in sulfuric acid.
Over 0.5
mm/year
Under 0.1 mm/year
20
40
60
80
Acid concentration, wt%
Fig. 7 — Iso-corrosion diagram for
C-2000 alloy in sulfuric acid.
Over 0.5 mm/year
0.1 to 0.5 mm/year
Boiling point curve
80
Under 0.1 mm/year
40
80
20
40
60
Acid concentration, wt%
Fig. 4 — Iso-corrosion diagram for
B-3 alloy in phosphoric acid.
and from plant to plant due to variations in the composition of phosphate rock. In addition, the corrosivity of a particular solution can vary with time of
storage, as a result of impurity interactions. Impurities include fluoride ions, chloride ions, silica,
aluminum, iron (which serves to increase the oxidizing potential of the acid), calcium, and sodium.
Instead, it is customary to test alloys at specific
concentrations and temperatures typical of the
production process. Within the fertilizer industry
it is also customary to define phosphoric acid concentrations in terms of P2O5 content. Corrosion
rates for G-30 and G-35 alloys in several concentrations of P2O5 (supplied by plants in Florida) at
121°C are shown in Fig. 3.
Pure phosphoric acid, which is added to foods,
is made by oxidizing elemental phosphorus derived from phosphate rock, then adding water.
This type of phosphoric acid is much less corrosive than the fertilizer grade, as indicated by the
250
120
Boiling point curve
Temperature, °C
G-35 in acid 1
G-35 in acid 2
G-30 in acid 1
G-30 in acid 2
0.4
80
Fig. 5 — Iso-corrosion diagram for
C-2000 alloy in phosphoric acid.
iso-corrosion diagrams for B-3 and C-2000 alloys
in Fig. 4 and 5. Indeed, only at high concentrations
and temperatures do the Ni-Mo, Ni-Cr, and NiCr-Mo alloys exhibit corrosion rates in excess of
0.1 mm/y in pure phosphoric acid.
Sulfuric acid
Sulfuric acid is one of the most important industrial chemicals. Not only is it used in the manufacture of fertilizer-grade phosphoric acid, but
also it serves as a catalyst in the petroleum industry and as a reactant in the production of detergents, polymers, and pigments.
Provided that the acid is pure, the Ni-Mo materials are the most resistant nickel alloys; second
best are the Ni-Cr-Mo alloys. Impurities of an oxidizing nature are extremely detrimental to the function of the Ni-Mo materials, whereas the Ni-Cr-Mo
alloys generally benefit from such impurities. Isocorrosion diagrams for B-3, C-2000, 400, and 625 alloys are shown in Fig. 6 to 9. A 0.1 mm/y line comparison between C-2000 alloy, 316L, 254SMO alloy,
and 20Cb-3 alloy (designed especially for service
in sulfuric acid) is shown in Fig. 10.
Sulfuric acid is reducing to the nickel alloys at
concentrations up to about 60 wt% (meaning that
the cathodic reaction is hydrogen evolution). At
higher concentrations, other cathodic reactions
are possible, and some materials exhibit sharp
dips in performance. Concentrated, industrialgrade sulfuric acid is known to contain impurities of an oxidizing nature; indeed, at temperatures in excess of about 90°C (194°F), the oxidizing
potential of this acid is beyond the scope of the
chromium-bearing nickel alloys.
250
250
Boiling point curve
150
Over 0.5
mm/year
100
0.1 to 0.5 mm/year
50
Under 0.1 mm/year
20
40
60
80
Acid concentration, wt%
Fig. 8 — Iso-corrosion diagram for
alloy 400 in sulfuric acid.
46
200
150
100
0.1 to 0.5 mm/year
Over 0.5 mm/year
50 0.1 to 0.5 mm/year
Under 0.1 mm/year
316L
254SMO
20Cb-3
C-2000
BP curve
200
Boiling point curve
Temperature, °C
Temperature, °C
200
Temperature, °C
250
150
100
50
20
40
60
80
Acid concentration, wt%
Fig. 9 — Iso-corrosion diagram for
alloy 625 in sulfuric acid.
20
40
60
Acid concentration, wt%
80
Fig. 10 — Comparison of 0.1 mm/year lines of several
nickel alloys in sulfuric acid.
ADVANCED MATERIALS & PROCESSES/SEPTEMBER 2007
A nickel alloy designed for such situations is
a high-silicon Ni-Cr material known as D-205
alloy. However, the mechanical properties of D205 weldments are such that it can only function
safely in the form of plate heat exchangers.
Caustic alkalis
Sodium hydroxide (known also as caustic soda)
and potassium hydroxide (known also as caustic
potash) are widely used chemicals. Applications
include the manufacture of soap, paper, and aluminum. They also serve to neutralize acids, especially in the petrochemical industries. Molten
sodium hydroxide functions to descale stainless
steels and other alloys in the metals
industry.
As indicated earlier, the commercially pure nickels are the premier
materials for service in caustic alkalis. They are resistant over very
wide ranges of concentration and
temperature. Alloys 400 (from the
Ni-Cu group) and 600 (from the NiCr group) are also favored in caustic
alkalis. For situations involving
caustic alkalis on the one hand and
chlorinated compounds on the other,
the Ni-Cr-Mo alloys are candidates.
However, it has recently been discovered that high-molybdenum nickel
alloys are susceptible to “caustic
dealloying” in strong, hot, caustic solutions. This means that elements
other than nickel are selectively
leached from surfaces. Field testing
is therefore important prior to
placing Ni-Cr-Mo materials into
service in caustic alkalis.
the chemical process industries, several other industries take advantage of their corrosion behavior. These include Food: Commercially pure
nickels are easy to form and provide inherent resistance to mild corrosives; Marine: Ni-Cu alloys
enable resistance to corrosion and cavitation erosion; Power: Ni-Cr-Mo alloys line flue gas desulfurization systems; and Metal finishing: Ni-Cr
and Ni-Cr-Fe alloys serve as pickling fixtures.
For more information: Paul Crook is Product R&D
Manager at Haynes International, 1020 W. Park Avenue,
Kokomo, IN 46904-9013; tel: 765/456-6241; pcrook@
haynesintl.com; www.haynesintl.com.
Welding nickel alloys
Three processes are typically
chosen to weld the nickel alloys. For
sheets and plate root passes, gas tungsten arc (TIG) welding is favored. For
plate welds, the gas metal arc (MIG)
process is preferred. For field welding,
the shielded metal arc process with
coated electrodes is favored.
To minimize the precipitation of
second phases in regions affected by
the heat of welding, a maximum interpass temperature of 90°C is recommended for the nickel alloys.
Also, welding of cold-worked materials is strongly discouraged, since
they sensitize more quickly and induce residual stresses. A full solution anneal, followed by water
quenching, is recommended for
cold-worked structures prior to
welding.
Applications
While the corrosion-resistant
nickel alloys are used primarily in
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47