Copper Alloys Stress Corrosion Cracking

Application Data Sheets – Copper Alloys

Stress Corrosion Cracking

What Is Stress Corrosion Cracking (SCC)

Stress corrosion cracking is a method of failure of metal parts. It is usually characterized by severe cracking started by a combination of chemical attack and material stress. It can lead to complete failure of parts without warning and can lead to serious risks.

stress corrosion cracking

History of Stress Corrosion Cracking (SCC)

This method of failure was first observed in items made of brass in the warm summers was termed ‘season cracking’.

Whilst some brass specimens of failure were observed before 1900, it was during the first world war where brass shell casings were observed to crack when stored near horses that the effect of ammonia ( from the horse urine ) on brass pressings was firmly identified.

stress corrosion cracking

How to reduce the risk of stress corrosion cracking

Cracking in copper alloys and brasses is generally as a result of internal residual stresses from manufacture adding to any service tensile stress. Stress relief annealing after manufacture reduces these risks. Understanding the effect of chemical attack, particularly from traces of chemicals used in the manufacturing process, will greatly reduce the risk of service failure.

Grades with higher resistance stress corrosion cracking

Generally the higher copper content alloys including ‘pure’ coppers are less prone to this type of cracking. Very high stresses of up to 50% of the yield strength and long exposure times are needed to create cracking in ‘pure’ copper materials.

Metals most susceptible to SCC

Brass alloys with a copper content below 70% are most susceptible to stress corrosion cracking scc. Historically known as Muntz metal, it was in these alloys that stress corrosion cracking scc was first observed. Service failure of pure coppers is extremely rare and limited to cases of very high service stresses. Failure of coppers due to internal stresses alone is unknown.

stress corrosion cracking

How to repair stress corrosion cracking

Generally it is best to avoid stress corrosion cracking scc by avoiding the types of chemicals that can lead to corrosion cracking. Manufactured parts that exhibit this type of cracking usually cannot be repaired as the risk of further catastrophic failure is too great.

How to avoid stress corrosion cracking (SCC)

Formed parts should be stress relieved after pressing, in most cases service stresses alone are not high enough to cause cracking. Parts processed with industrial chemicals should be cleaned and washed with water or other unreactive agents. Care should be taken to design parts without crevices that might catch residual chemicals and making washing difficult.

Facts in brief about stress corrosion cracking

Cracking normally occurs between grain boundaries and often displays many branched cracks. Further exposure can cause the remaining metal to become brittle and lead to complete failure. Cracking is normally caused by a combination of residual trace chemicals and internal material stresses.

What chemicals cause SCC

The most common chemical exposure for brasses and coppers than can cause stress corrosion cracking is ammonia; a compound found in urine. Some chemical agents such as nitrites can decay into ammonia. Brass has been shown to corrode in aqueous solutions of ammonia, sodium nitrite, citrates, sulfur dioxide and sulfate solutions (Davis JR. Forging and Extrusion. In: Davis JR, ed. Copper and Copper Alloys . Materials Park: ASM International; 2001. p. 220-222.) Stress corrosion cracking of brasses has been shown to occur most severely in ammonia vapour. The amount of ammonia and of copper ions in the form of copper sulphate has been found to increase the corrosion rate of brass.

Formation of SCC cracking and effects of the environment

Macroscopically, cracks can occur along geometric discontinuities such as edges of the pressed parts and of any defects or discontinuities within the metal.

stress corrosion cracking

Microscopically, it has been shown that trans-granular and inter-granular cracking of the metal can occur in corrosive environments.

The type of microscopic cracking is dependent on the amount and type of corrosion product present, the accessibility of oxidizing agents, or copper ions present, the pH of the environment and the amount of residual stress in the brass products. These factors will either promote diffusion of ionic species into the metal or emphasize the presence of defects within the parts.

stress corrosion cracking

Identifying stress corrosion cracking

Any unexpected cracking of formed parts will normally be caused by stress corrosion. In some cases the source of the chemical attack is not obvious. In copper and brass materials environmental factors are not normally the source; ammonia is usually created in an industrial setting.

Normal service life

By carefully avoiding contamination with the known causes of stress corrosion cracking formed parts of copper and copper alloys can be expected to provide the normal extended service life associated with these alloys.

High stresses and the presence of one specific chemical ( ammonia ) are normally required to produce service failure of copper and copper alloys due to stress corrosion cracking.

Table 24.4 Stress Corrosion Cracking of Copper Alloys in the Atmosphere

Two Industrial Sites (New Haven, Brooklyn) and One Marine Site (Daytona Beach) [17]

CDA
Temper
Time to Failure
Crack Morphologya
New Haven
Brooklyn
Daytona Beach
New Haven
Brooklyn
Daytona Beach
Ident
% Cold Rolled
110 37% NFd 8.5 yrs NF 8.5 yrs NF 8.8 yrs
194 37% NF 8.5 yrs NF 8.5 yrs NF 8.8 yrs
195 90% NF 3.2 yrs NF 3.1 yrs NF 3.1 yrs
230 40% NF 8.5 yrs NF 8.5 yrs NF 8.8 yrs
260 50% NF 35-47 days 0-23 days NF 2.7 yrs I I
353 50% 51-136 days 70-104 days NF 2.7 yrs T + (I) T + (I)
405 50% NF 2.7 yrs NTe NT
411 50% NF 2.7 yrs NT NT
422 37% NF 8.5 yrs NF 8.5 yrs NF 8.8 yrs
425 50% NF 2.7 yrs NT NT
443 10% NF 2.7 yrs NF 2.7 yrs NF 2.7 yrs
40% 51-95 days 41-70 days NF 2.7 yrs T T
40% 51-67 days 33-49 days NF 2.7 yrs T T
510 37% NF 8.5 yrs NF 8.5 yrs NF 8.8 yrs
521 37% NF 5.7 yrs NF 5.7 yrs NF 5.7 yrs
619 40%, 9% ? phasec NF 8.5 yrs NF 8.5 yrs NF 8.8 yrs
40%, 95% ? phase NF 8.5 yrs NF 8.5 yrs NF 8.8 yrs
638 50% NF 5.7 yrs NF 5.7 yrs NF 5.7 yrs
672 annealed 0-30 days 0-134 days NF 3.1 yrs I I
50% 0-30 days 0-22 days 18-40 days I I I
687 10% 517-540 days 2.3 – NF 2.7 yrs NF 2.7 yrs T T
40% 221-495 days 311-362 days NF 2.7 yrs T T
40% + orderedb 216-286 days 143-297 days NF 2.7 yrs T T
688 10% NF 2.7 yrs NF 2.7 yrs NF 2.7 yrs
40% 4.7-NF 6.4 yrs 2.7-NF 6.4 yrs NF 6.4 yrs T T
40% + orderedb NF 2.7 yrs NF 2.7 yrs NF 2.7 yrs
706 50% NFb 2.2 yrs NF 2.3 yrs NF 2.2 yrs
725 40% NF 2.2 yrs NF 2.3 yrs NF 2.2 yrs
752 annealed NF 3.2 yrs NF 3.1 yrs NF 3.1 yrs
25% NF 3.2 yrs NF 3.1 yrs NF 3.1 yrs
50% NF 3.2 yrs NF 3.1 yrs NF 3.1 yrs
762 annealed 171-NF 3.2 yrs 672-NF 3.1 yrs NF 3.1 yrs T T
25% 142-173 days 236-282 days NF 3.1 yrs T T
50% 142-270 days 236-282 days NF 3.1 yrs T T
766 38% 127-966 days 197-216 days 754-NF 8.8 yrs T T T
770 annealed 731-1003 days 337-515 days NF 3.1 yrs T T
38% 137-490 days 196-518 days 596-1234 days T T T
50% 153-337 days 489-540 days 692-970 days T T T
782 50% 23-48 days 26-216 days 236-300 days T + (I) T + (I) T

aI = intergranular, T = transgranular, and parentheses indicates minor mode.
bHeated 400°F, 1/2 hour.
cNormal Structure for this alloy.
dNF = No failures in time specified.
eNT = Not tested

Approximate Composition of Alloys

CDA Cu Zn P Sn Pb Others
110
Electrolytic Tough Pitch Copper
99.90%
0.04% O
194
High Strength Modified Copper
97.5% 0.13 0.03%
2.4% Fe
195
Strescon
97.0% 0.10% 0.6%
1.5% Fe, 0.80% Co
230
Red Brass 85%
85% 15.0%
260
Cartridge Brass 70%
70.0% 30.0%
353
High Leaded Brass
62.0% 36.2% 1.8%
405
High Conductivity Bronze
95% 4% 1%
411
Lubaloy
91% 8.5% 0.5%
422
Lubronze
87.5% 11.4% 1.1%
425 88.5% 9.5% 2.0%
443
Arsenical Admiralty Brass
71.0% 28.0% 1.0%
0.05% As
510
Phosphor Bronze 5%
95% 0.1% 5.0%
521
Phosphor Bronze 8%
92.0% 0.1% 8.0%
619 86.5%
4.0% Fe, 9.5% Al
638
Coronze
95.0%
2.8% Al, 0.40% Co, 1.8% Si
672
687
Arsenical Aluminium Brass
77.5% 20.5%
2.0% Al, 0.1% As
688
Alcoloy
73.5% 22.7%
3.4% Al, 0.40% Co
706
90-10 Copper Nickel
88.7%
1.3% Fe, 10.0% Ni
725
Tin Bearing Copper Nickel
88.2% 2.3%
9.5% Ni
752
Nickel Silver, 65-18
65.0% 17.0%
18.0% Ni
762 59.0% 29.0%
12.0% Ni
766
770
Nickel Silver, 55-18
55.0% 27.0%
18.0% Ni
782 65.0% 25.0% 2.0%
8.0% Ni, 2% Pb

Note: many of these alloys are proprietary or obsolete. They may not be available.

Source of stress corrosion cracking data: J.M.Popplewell & T.V.Gearing Corrosion 32 p279, 1975

The technical advice and recommendations made in this Product Data Sheet should not be relied or acted upon without conducting your own further investigations, including corrosion exposure tests where needed. Please consult current editions of standards for design properties. Austral Wright Metals assumes no liability in connection with the information in this Product Data Sheet. Austral Wright Metals supplies a comprehensive range of stainless steels, copper alloys, nickel alloys and other high performance metals for challenging service conditions. Our engineers and metallurgists will be pleased to provide further data and applications advice.
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