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]

Time to Failure
Crack Morphologya
New Haven
Daytona Beach
New Haven
Daytona Beach
% Cold Rolled
11037%NFd 8.5 yrsNF 8.5 yrsNF 8.8 yrs
19437%NF 8.5 yrsNF 8.5 yrsNF 8.8 yrs
19590%NF 3.2 yrsNF 3.1 yrsNF 3.1 yrs
23040%NF 8.5 yrsNF 8.5 yrsNF 8.8 yrs
26050%NF 35-47 days0-23 daysNF 2.7 yrsII
35350%51-136 days70-104 daysNF 2.7 yrsT + (I)T + (I)
40550%NF 2.7 yrsNTeNT
41150%NF 2.7 yrsNTNT
42237%NF 8.5 yrsNF 8.5 yrsNF 8.8 yrs
42550%NF 2.7 yrsNTNT
44310%NF 2.7 yrsNF 2.7 yrsNF 2.7 yrs
40%51-95 days41-70 daysNF 2.7 yrsTT
40%51-67 days33-49 daysNF 2.7 yrsTT
51037%NF 8.5 yrsNF 8.5 yrsNF 8.8 yrs
52137%NF 5.7 yrsNF 5.7 yrsNF 5.7 yrs
61940%, 9% ? phasecNF 8.5 yrsNF 8.5 yrsNF 8.8 yrs
40%, 95% ? phaseNF 8.5 yrsNF 8.5 yrsNF 8.8 yrs
63850%NF 5.7 yrsNF 5.7 yrsNF 5.7 yrs
672annealed0-30 days0-134 daysNF 3.1 yrsII
50%0-30 days0-22 days18-40 daysIII
68710%517-540 days2.3 – NF 2.7 yrsNF 2.7 yrsTT
40%221-495 days311-362 daysNF 2.7 yrsTT
40% + orderedb216-286 days143-297 daysNF 2.7 yrsTT
68810%NF 2.7 yrsNF 2.7 yrsNF 2.7 yrs
40%4.7-NF 6.4 yrs2.7-NF 6.4 yrsNF 6.4 yrsTT
40% + orderedbNF 2.7 yrsNF 2.7 yrsNF 2.7 yrs
70650%NFb 2.2 yrsNF 2.3 yrsNF 2.2 yrs
72540%NF 2.2 yrsNF 2.3 yrsNF 2.2 yrs
752annealedNF 3.2 yrsNF 3.1 yrsNF 3.1 yrs
25%NF 3.2 yrsNF 3.1 yrsNF 3.1 yrs
50%NF 3.2 yrsNF 3.1 yrsNF 3.1 yrs
762annealed171-NF 3.2 yrs672-NF 3.1 yrsNF 3.1 yrsTT
25%142-173 days236-282 daysNF 3.1 yrsTT
50%142-270 days236-282 daysNF 3.1 yrsTT
76638%127-966 days197-216 days754-NF 8.8 yrsTTT
770annealed731-1003 days337-515 daysNF 3.1 yrsTT
38%137-490 days196-518 days596-1234 daysTTT
50%153-337 days489-540 days692-970 daysTTT
78250%23-48 days26-216 days236-300 daysT + (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

Electrolytic Tough Pitch Copper
0.04% O
High Strength Modified Copper
2.4% Fe
1.5% Fe, 0.80% Co
Red Brass 85%
Cartridge Brass 70%
High Leaded Brass
High Conductivity Bronze
Arsenical Admiralty Brass
0.05% As
Phosphor Bronze 5%
Phosphor Bronze 8%
4.0% Fe, 9.5% Al
2.8% Al, 0.40% Co, 1.8% Si
Arsenical Aluminium Brass
2.0% Al, 0.1% As
3.4% Al, 0.40% Co
90-10 Copper Nickel
1.3% Fe, 10.0% Ni
Tin Bearing Copper Nickel
9.5% Ni
Nickel Silver, 65-18
18.0% Ni
12.0% Ni
Nickel Silver, 55-18
18.0% Ni
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.