hydrogen in alumimium

7/31/2019 Hydrogen in Alumimium

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Hydrogen in Aluminium

Solubility, Characterisation and Removal

R. N. Chouhan


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Principles of solubility and removal of hydrogen

Sources of hydrogen

Defects caused by presence of hydrogenDetection of Hydrogen

Removal

Contents


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Principles

Solubility

Hydrogen Precipitation

Principles of hydrogen removal


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Solubility

3H2O + 2Al 6H + Al2O3

the equilibrium reaction for dissolution ofhydrogen in aluminium can be expressed as

H2 (gas) [H] (metal)

For above equation equilibrium constantK is given by

KH = [activity of hydrogen]

(partial pressure of H2)1/2


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If the dissolved gas is sufficiently dilute that it obeys

Henrys law

KH = [Wt%H] / PH21/2

This expression is called Sieverts Law

above expression can be simplified as

Log(wt%H) = - A/T + Log(PH2

) +

Constant

Sieverts Law


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Empirical relationship for solubility of hydrogen

log CH

= -2761/T + 2.768 (L)

log CH= -2580/T + 1.399 (S)

Where, CH represents cc of hydrogen gas/100 g of

aluminium at 1 atm and T is the absolute temperature


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0

0.5

1

1.5

2

2.5

400 500 600 700 800 900

Temperature (C)

Hydroge

n(ml/100g)

Solubility of hydrogen in aluminium at 1 atm


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Ce, Li, Th, & Ti increases

Si, Cu, & Sn decreases, and

Fe, & Cr have a marginal effect on solubility

Effects of other alloying elements


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Hydrogen Precipitation

Concentration gradient ahead of the solidification front

Co = Cl

Cs

= 0.05


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Gas nucleation

For a gas bubble to nucleate remain stable or grow

the pressure inside the bubble must be equal to or

exceed the sum of the hydrostatic pressure, the

pressure of the atmosphere above molten metal

and surface tension forces

PH2 Ph + Pa + Surface Tension forces

Where PH2 =equilibrium pressure of hydrogen

Ph = hydrostatic pressure of metal

Pa = atmospheric pressure


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Gaseous cap formed on a solid substrate is shownalong with the various interfacial energies which act.

cos = ( SL - SG) / LG

Gas nucleation

LGcos + SG = SL


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Gas pores nucleated due inclusion which is pushedahead of the solid-liquid interface


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Bubbles trapped between dendrite arms


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Forces acting on existing bubble

Pintr

Pext


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For mechanical equilibrium it can be simplified as

Pint Pext =2T/r

where T = surface tension 1 N/m2 for aluminium

So if Pext = 0.1 atm =0.1 x105 N/m2 and neglecting Pext

r2T/ Pint 0.5 m

Equilibrium pore size


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Spherical pores


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Typical porosities


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The principle of hydrogen removal

KH = [Wt%H] / PH21/2

Log(wt%H) = - A/T + Log(PH2) +Constant

Application of vacuum

Gas purging


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H2

H2

P3>P2

P2>P1

P1>0

P1 = 0

Movement ofgas purge

bubble

Atmosphere

Melt containingdissolvedHydrogenH2

H2

H2

H2

H2

H2

The principle of gas purging

S f


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Furnace atmosphere, which contains water vapour

in addition to some H2

Moisture from refractories, dirty skimmers, and

other furnace tools

Hydrated corrosion products, which form part of the

charge, such as

Weathered ingot and scrap

Sources of hydrogen

S f h d


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Oil contaminated turnings, chips, or scrap

Damp fluxes, and

Oil and hydroxide coating on metallic

sodium used for modifying AlSi alloys

Metal mould reaction

Metal turbulence, improper gating

Sources of hydrogen


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Dissolved hydrogen in molten aluminium results in

porosity, the size and shape of which is dependent on

Composition of the alloy,

Its solidification characteristics

Microstructural featuresPresence of porosity nucleation sites.

Interdendritic porosity, which is encountered when

hydrogen content is sufficiently high ( > 0.15 cc/100 g)

D f t d b f h d


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Porosity

Low toughness

Inferior surface finish

Lower weldability

Leakage of pressurised castings in serviceBright flakes in forgings

SCC in Al-Zn-Mg-Cu alloys

H2 embrittlement

Defects caused by presence of hydrogen


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Detection of Hydrogen

Various quantitative and semiquantitative methods for the

measurement of the hydrogen content in aluminium and its

alloys featuring various degrees of sophistication have been

evolved. The choice of the appropriate technique is often

difficult since it can be affected by several factors, viz.

Stage of production,

Speed of analysis,

Detection range,

Capital investmentDesired accuracy,

Ease of operation etc.

T h i f S lid S l


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Techniques for Solid Samples

Vacuum Tin Fusion

Vacuum Fusion

Nitrogen carrier Fusion

Hot Vacuum Subfusion Extraction


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Vacuum Tin Fusion

The technique involves fusing of the aluminium alloy

sample in a molten tin bath at about 500C and

extraction of the hydrogen evolved during the fusion

process is delivered into an evacuated vacuum

system. Depending on the alloy and its solubility in

tin, complete solution of the sample requires 1-1.5

hrs. The apparatus consists of a furnace sectionwhere the hydrogen is extracted, a section for

collection and measurement of gas evolved from the

sample and a section for the analysis of the collected

gas . The collected gas is analysed for hydrogen witha mass spectrometer or by diffusion through a

palladium tube heated to 600c. Its accuracy is 0.07-

0.08 cc/100 g for pure aluminium

V F i


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Vacuum Fusion

This technique involves melting theprepared solid sample in a thoroughly

degassed boron nitride, graphite, or

alumina crucible in a high vacuum (10-3). A

rapid evaluation of gas occurs as themetal is melted. This reaches completion

after 3-5 mins. The hydrogen evolved is

extracted and separated from background

gas by a palladium tube or by analysingwith a mass spectrometer

Nit i F i


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Nitrogen carrier Fusion

The sample is placed in an outgassed graphite crucible in a

silica (quartz) tube. The graphite crucible is baked out anddegassed at a high temperature (~2000C) by induction

heating for 3-5 mins. The inside of this tube is isolated from

the ambient atmosphere by a flow of nitrogen. The sample is

introduced into the crucible. During surface contamination

removal the surface temperature of the sample is reported to be

at 400C-480C for 60 seconds. The sample is allowed to cool

down for 5 mins before commencement of the extraction. The

system is sealed and the sample is melted by induction heating

by raising the plate current and gas is extracted for 3-5 mins.

Hydrogen is extracted by diffusion from the liquid sample into

the nitrogen stream and is then detected by katharometer



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A dry-machined cylindrical sample, is heated in an

evacuated clear silica tube to a temperature beloweutectic or solidus temperature, until a definite

endpoint is obtained on the gas evolution or

extraction curve recorded with a strip chart recorder

via a Pirani, Baratron or ionization gauge. Theextraction time varies from 1-2 hrs. When extraction

is complete, the gas is subjected to a simple

analytical test at constant volume with either the

mass spectrometer or by heating the palladium tube

to diffuse the hydrogen out.

T h i f M lt M t l S l


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Techniques for Molten Metal Samples

Straube-Pfeiffer (Vacuum Gas) Test

The Initial Bubble Test

Recirculating Gas Methods

S b Pf iff (V G ) T


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Straube-Pfeiffer (Vacuum Gas) Test



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Sample of molten aluminium contained in anelectrically heated crucible is placed in a closed

chamber, and vacuum is gradually applied until

the first bubble is observed at the molten metal

surface. The pressure and temperature at whichthe first bubble appears are recorded. A

nomograph relating pressure, temperature and

hydrogen solubility of the alloy being tested is

used to obtain the hydrogen content.

R i l ti G M th d


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Recirculating Gas Methods

It operates on the principle of monitoring hydrogenactivity developed in a small quantity of inert gas

continuously recirculated through the molten metal

under test until the gaseous hydrogen diffused into the

purged gas bubble is in equilibrium with the solute

hydrogen in the molten metal in accordance withSieverts law.

If the solubility of hydrogen in the alloy at a given

temperature and pressure is known then hydrogen C

content can be obtained from:

C = STP + (Pi/P)1/2


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ALCOA Telegas IITM Instrument

Diff t R i l ti G M th d


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ALCOA Telegas IITM Instrument

QRG Test Unit

SLM Hydrogen Determinator

Different Recirculating Gas Methods

Sample preparation and handling


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Sample preparation and handling

Preparation and handling of samples are very critical in

hydrogen content determination because contamination ofthe sample is one of the principle sources of spurious

hydrogen in the determination of the hydrogen content in the

solid alloy samples. In majority of the cases, surface

preparation determines the reproducibility of the test resultsand thus should be standardised Samples by dry turning for

direct method to be prepared with caution and immediately

tested. After 24 hrs remachining is required. For liquid

samples (indirect method), preheated mould permitting rapid

solidification with smooth feeding is required to prevent lossof hydrogen and formation of porosity


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Removal

Various methods of removing H2 from melt existsuch as gas purging, vacuum and flux

degassing. The most common methods (fig. 9)

rely on bubbling gases through the melt to carry

H2 to the surface. The efficiency of this processdepends mainly on the size of the bubble

produced- small bubbles give better efficiency.

The types of gases used can be split into two

categories, reactive and inert.

R ti


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The gases react with the melt in various ways.They usually contain Cl2, Cl2 compounds or Cl2

mixtures e.g.

Cl2Ar with 50% Cl2 Mixture

N2 with 10% Cl2 Mixture

Freon CompoundHexachloroethane Compound

Reactive gases

I t


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Nitrogen or argon is used to remove hydrogenfrom the melt. There is much less removal of

solid particles by floatation. Some removal of

Na and Mg may occur but this process can be

carried out after SrAl modification.

Inert gases

Diff t d i t h i


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Different degassing techniques

Natural degassing

Vacuum degassing

Ultrasonic treatment

Gas purging

Tablet/Flux degassing

Gas p rging


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Gas purging

Rotary degassing


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Rotary degassing


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Comparison of degassing techniques


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Lance Degassing

Bubble Size: 2-3 cmDispersion: Poor

Efficiency: Poor-Med

Consistency: Poor-Med.

Rotary Degassing

Bubble Size: 2-5 mmDispersion: Very Good

Efficiency: High

Consistency: Good

Porous Plug

Bubble Size: 2-10 mmDispersion: Fairly Poor

Efficiency: Med.-Good

Consistency: Medium

Tablet Degassing

Bubble Size: VariableDispersion: Poor

Efficiency: Variable

Consistency: Poor

Comparison of degassing techniques


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hydrogen in alumimium

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