Sha W., Malinov S. Titanium Alloys: Modelling of Microstructure, Properties and Applications

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Surface gas nitriding: mechanical properties, morphology 483

17.3.3 Microhardness profiles

The microhardness is very high near the surface and falls gradually with the

distance from the surface. There is no significant difference in the microhardness

values for different initial roughness.

(g) Z scale = 2 µm,

= 0.036 µm

0 20.0 40.0 60.0

µm

60.0

40.0

20.0

(h) Z scale = 2 µm,

= 0.265 µm

0 20.0 40.0 60.0

µm

60.0

40.0

20.0

17.21

Continued

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Titanium alloys: modelling of microstructure484

µm

(a)

µm

(b)

µm

(c)

17.22

3D images of the surface morphology of polished Ti-6Al-2Sn-

4Zr-2Mo (a, c, e and g) before and (b, d, f and h) after gas nitriding at

different temperatures and for different periods of time. (a) and (b)

950 °C, 1 hour; (c) and (d) 950 °C, 3 hours; (e) and (f) 950 °C, 5 hours;

(g) and (h) 1050 °C, 5 hours. The

scale for (b) is 4 µm/division and

for all the rest is 2 µm/division.

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Surface gas nitriding: mechanical properties, morphology 485

µm

(d)

µm

(e)

µm

(f)

17.22

Continued

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Titanium alloys: modelling of microstructure486

The thickness of the nitrided layer can be estimated from the microhardness

profiles (see Section 17.1). As was discussed in that section, it can be supposed

that the nitrided layer ends where the microhardness value approaches the

core microhardness. For Ti-6Al-2Sn-4Zr-2Mo, gas nitrided at 950 °C for

5 hours, nitrided layers of about 200 µm have been obtained.

17.3.4 Summary

Gas nitriding significantly increases the surface roughness of Ti-6Al-2Sn-

4Zr-2Mo titanium alloy, up to the order of 25 times, except for the substrate

with high initial roughness already. The surface roughness after nitriding

depends on the initial surface roughness of the material. It increases with

increase of the initial roughness. The surface roughness increases with increase

of the nitriding time, but there is no significant change of the surface roughness

with increase of the temperature from 950 to 1050 °C. The increase of the

µm

(h)

17.22

Continued

µm

(g)

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Surface gas nitriding: mechanical properties, morphology 487

surface roughness after gas nitriding is caused by the formation of titanium

nitride and titanium oxide on the surface of the materials. There is no significant

influence of the different initial surface roughness on the microhardness and

the thickness of the nitrided layer after nitriding.

17.4 Corrosion behaviour

One of the biggest advantages of titanium alloys is their good corrosion

resistance. Much research and testing have been carried out to assess the

corrosive behaviour of titanium alloys in different environments before and

after different types of thermo-chemical surface treatments, which can either

improve or worsen their corrosion resistance properties. The data in the

literature is quite diverse and inconsistent in this area. This section will

quantify the effect of the corrosive media and test temperature on the corrosion

behaviour of Ti-6Al-2Sn-4Zr-2Mo and Ti-8Al-1Mo-1V before and after the

surface thermo-chemical treatment of gas nitriding.

⇐

•

{100}

←

{101}

•

/←

• α-Ti(N) ⇓ TiN ↓ TiO

1050/5

950/5

950/3

950/1

•

/←

←

•

←

⇐

←

•

←

•

⇐

←

•

⇐

•

/←

•

/←

←

•

←

⇐

←

•

←

⇐

←

•

/←

←

•

←

⇐

←

•

⇐

{111}

•

{002}

←

{200}

•

{101}

←

{111}

⇐

{200}

←

{210}

←

{211}

•

{102}

←

{220}

⇐

{220}

•

{110}/

←

{002}

←

{310}

←

{301}

•

{103}/

←

{112}

⇐

{311}

40 50 60 70

2θ (°)

Intensity (c.p.s.)

400

100

900

400

100

900

400

100

1600

400

17.23

X-ray diffraction patterns of Ti-6Al-2Sn-4Zr-2Mo after gas

nitriding at 950 °C for 1, 3 and 5 hours and at 1050 °C for 5 hours.

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Titanium alloys: modelling of microstructure488

17.4.1 Basic principles of titanium alloys corrosion

General corrosion of titanium alloys

Titanium, like any other metal, is subject to corrosion in certain environments.

The types of corrosion that have been observed on titanium may be classified

under the following general headings: general corrosion, crevice corrosion,

stress corrosion cracking, anodic breakdown pitting, and galvanic corrosion.

This section (Section 17.4) is aimed specifically at the general corrosion

resistance of titanium alloys before and after surface gas nitriding. General

corrosion is characterised by a uniform attack over the entire exposed surface

of the metal. The severity of this type of attack can be expressed by the

corrosion rate. General corrosion in aqueous media may take the form of

mottled or severely roughened metal surfaces. When titanium is fully passive,

corrosion rates are typically around 0.04 mm/yr, much lower than 0.13 mm/yr

maximum rate accepted by engineers. This very low corrosion rate is attributed

to a titanium oxide film on the surface. The exact process will be explained

next in this section. As a result, titanium is often designed with a zero

corrosion allowance in passive environments. However, in some environments,

(c)

100 µm

(d)

500 µm

(b)

100 µm

(a)

100 µm

17.24

Microstructure of Ti-6Al-2Sn-4Zr-2Mo. (a) 950 °C, 1 hour;

(b) 950 °C, 3 hours; (c) 950 °C, 5 hours; (d) 1050 °C, 5 hours.

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Surface gas nitriding: mechanical properties, morphology 489

titanium may experience an oxide growth, characterised by a coloured surface

and slight weight gain.

General corrosion really becomes a concern in reducing acid environments,

especially if the acidity and temperatures begin to rise. In strong or hot

reducing acids, the oxide film on the surface will deteriorate and break

down, so leaving the metal surface susceptible to corrosion.

Mechanism of corrosion resistance

The excellent corrosion resistance of titanium alloys is due to the formation

of a very stable, continuous, highly adherent, and protective oxide film on

the metal surfaces. Because titanium metal itself is highly reactive and has

an extremely high affinity for oxygen, these beneficial surface oxide films

form spontaneously and instantly, when fresh metal surfaces are exposed to

air, or moisture. In fact, a damaged oxide film can generally heal itself

instantaneously if at least traces (that is, parts per million) of oxygen or

water (moisture) are present in the environment.

The composition of this film varies from TiO

at the surface to Ti

and

TiO at the metal interface. Oxidising conditions promote the formation of

TiO

, so that in such environments the film is primarily TiO

. This film is

transparent in its normal thin configuration, and not detectable by visual

means. A study of the corrosion resistance of titanium is basically a study of

the properties of the oxide film, which is attacked only by certain substances,

under, for example, anhydrous conditions in the absence of a source of

oxygen.

Therefore, the successful non-corrosive properties of titanium and its

alloys can be expected in mildly reducing to highly oxidising environments,

in which protective oxide films spontaneously form and remain stable. Titanium

exhibits excellent resistance to atmospheric corrosion in both marine and

industrial environments.

The following is some background information on the effects of some

corrosive media.

Salt solutions

Titanium alloys are very resistant to almost all salt solutions over the pH

range of 3 to 11, and to temperatures that exceed their boiling point. Titanium

can withstand exposure to solutions of chlorides, bromides, iodides, sulfites,

sulfates, borates, phosphates, cyanides, carbonates, bicarbonates, and

ammonium compounds. The corrosion rate values for titanium alloys in this

variety of salt solutions are generally less than 0.03 mm/yr. Titanium alloys

are frequently used in many process streams, brines, and seawater, because

of their good resistance to the chlorides typically found in them.

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Titanium alloys: modelling of microstructure490

Reducing acids

This category includes hydrochloric, sulfuric, hydrobromic, hydriodic,

hydrofluoric, phosphoric, sulfamic, oxalic, and trichloroacetic acids. The

corrosion resistance of titanium alloys in reducing acid solutions is very

sensitive to the acid concentration, solution temperature and purity of the

acid solution, as well as to the alloy composition. With increase of the

temperature, or the concentration of the reducing acid solution, the protective

oxide film on the surface of the material may break down, which would

result in severe general corrosion. Various oxidising species can effectively

inhibit the corrosion of titanium in reducing acid environments, when present

in small concentrations. These inhibiting species often occur as natural process

stream constituents or contaminants, and do not need to be intentionally

introduced to achieve passivation.

Enhancing the corrosion resistance of titanium alloys

Basically, the only methods of increasing the corrosion resistance of titanium

in reducing environments are:

(i) increasing the surface oxide film thickness by anodising or thermal

oxidation

(ii) anodically polarising the alloy with a more noble metal, in order to

maintain the surface oxide film

(iii) applying precious metal surface coatings

(iv) alloying titanium with certain elements

(v) adding oxidising species to the reducing environment to permit oxide

film stabilisation.

Of these five methods, the last two are the most practical, and are widely

used in service.

Thermo-chemical nitriding of titanium alloys may also improve their

corrosion resistance. The problem after these types of treatments is that if the

nitrided surface has become pitted, the corrosion is higher for the nitrided

material, because of the potential difference between core and case. Many

researchers have studied the corrosion behaviour of titanium alloys before

and after nitriding (Gokul Lakshmi et al., 2002; Meletis, 2002; Takahashi

and Kimura, 2001).

Considering phase transformations taking place during the thermo-chemical

treatment of gas nitriding, one can expect that these newly-formed surface

layers would affect the corrosion properties.

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Surface gas nitriding: mechanical properties, morphology 491

17.4.2 Influence of the alloy composition and

surface gas nitriding

The two alloys discussed in this section are the near-α titanium alloys Ti-

8Al-1Mo-1V and Ti-6Al-2Sn-4Zr-2Mo. Normally, the additions of

molybdenum and vanadium improve the corrosion resistance of titanium

alloys, and aluminium content worsens it. These two alloys have different

alloy composition, and they would have different corrosion behaviour in

various media.

0.5

M NaCl

In 40 °C solution, Ti-8Al-1Mo-1V shows better corrosion resistance in

comparison with Ti-6Al-2Sn-4Zr-2Mo. At the higher medium temperature

of 80 °C, the relative corrosion resistance between the two alloys is the

opposite, and the weight loss increases under each condition except for Ti-

6Al-2Sn-4Zr-2Mo nitrided at 950 °C for 5 hours. Gas nitriding worsens the

corrosion resistance of Ti-6Al-2Sn-4Zr-2Mo at both medium temperatures,

but improves the corrosion resistance of Ti-8Al-1Mo-1V.

Despite the above differences, the corrosion resistance of these alloys in

0.5

M NaCl at both solution temperatures may be concluded as excellent,

with or without the nitriding, considering that the average weight loss values

never reach 0.0005 g/cm

after 1500 hours of corrosion test. The level of

weight loss in this salt solution is insignificant in comparison to the weight

loss in acidic media. These results are partly in contradiction to the results

given by Fleszar et al. (2000), where it was reported that plasma nitriding

significantly increased the corrosion resistance of Ti-6Al-3Mo-2Cr.

4.9

M HCl

The weight loss increases for all conditions with the time prolongation and

the increase of the temperature, except that there is no significant influence

of the temperature on the corrosion behaviour of the untreated Ti-8Al-1Mo-

1V alloy (Fig. 17.25).

The nitrided Ti-8Al-1Mo-1V shows, in general, better performance than

the nitrided Ti-6Al-2Sn-4Zr-2Mo. The weight loss values of the untreated

condition are similar for the two alloys at 20 and 40 °C. At 80 °C, Ti-8Al-

1Mo-1V shows a better performance in 4.9

M HCl in comparison to Ti-6Al-

2Sn-4Zr-2Mo, probably due to the advantageous combination of the alloying

elements in the former. These data are in contradiction with the results

obtained for Ti-6Al-4V by Galvanetto et al. (2002).

Gas nitriding worsens the corrosion resistance of these alloys and especially

of Ti-6Al-2Sn-4Zr-2Mo. The reducing acid solution is very aggressive and



Titanium alloys: modelling of microstructure492

Weight loss (g/cm

)

0.012

0.01

0.008

0.006

0.004

0.002

0 1000 2000 3000 4000

Time (h)

(a)

Ti-6242, nitrided

Ti-6242, untreated

Ti-811, nitrided

Ti-811, untreated

Weight loss (g/cm

)

0.012

0.01

0.008

0.006

0.004

0.002

0 500 1000 1500 2000 2500 3000

Time (h)

(b)

Ti-6242, nitrided

Ti-6242, untreated

Ti-811, nitrided

Ti-811, untreated

Weight loss (g/cm

)

0.02

0.016

0.012

0.008

0.004

0 500 1000 1500

Time (h)

(c)

Ti-6242, nitrided

Ti-6242, untreated

Ti-811, nitrided

Ti-811, untreated

17.25

Weight loss versus time of Ti-6Al-2Sn-4Zr-2Mo and Ti-8Al-1Mo-

1V, untreated and nitrided (950 °C for 5 hours), after holding them in

4.9

M HCl at (a) room temperature, (b) 40 and (c) 80 °C.

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