Corrosion failure of Zn–Al detonator housing

Swapan Kumar Das*, Dipak Kumar Bhattacharya

National Metallurgical Laboratory, Jamshedpur-831007, India

Received 22 April 2003; accepted 23 April 2003

Abstract

Some batches of detonator housings made up of Chromium plated Zn–Al alloy were found in an extensively cracked

condition after few months of storage at room temperature. An analysis of the failure showed that the cracks were dueto intergranular corrosion facilitated by segregation of lead at the grain boundaries. Improper chromium plating fur-ther aggravated the corrosion problem. This failure case emphasises the need for strict control of chemical composition

for components made from Zn–Al alloy and the process of Cr plating of the components.# 2003 Elsevier Ltd. All rights reserved.

Keywords: Zinc alloy; Intergranular corrosion; Segregation; Casting defects; Galvanic corrosion

1. Introduction

The three major areas of application of Zn metal are: (i) as a protective layer on steel items (galvanising),(ii) as an alloying element with copper in brass, and (iii) as variants of Zn–Al alloy in mass-produced diecast items. A comprehensive treatise on the properties and applications of Zn and its alloys can be found inRef. [1]. Two excellent die casting alloys as variants of Zn–4%Al alloy are used in lightly stressed appli-cations in many fields. They are known as Zamak 3 and 5 in USA and Mazak 3 and 5 in UK. It has beenknown for a long time [1] that there should be close control in these alloys on (i) the amount of aluminiumfor achieving the desired strength without losing ductility and on (ii) the trace elements such as lead, tinetc., which otherwise would segregate at the grain boundaries having almost nil solubility in zinc, andthereby seriously aggravate the intergranular corrosion resistance even under (humid) room temperatureconditions. Chromium plating further reduces the possibility of corrosion in Zn–Al alloy castings. Mag-nesium in small quantities plays a very positive role in mitigating intergranular corrosion.Even with this knowledge having been available for a long time, failures of Zn-Al cast components

are still reported [2]. The present paper based on a recent work emphasises the point that strict controlof chemical composition and proper chromium plating cannot be compromised if failures are to beavoided.

1350-6307/03/$ - see front matter # 2003 Elsevier Ltd. All rights reserved.

doi:10.1016/S1350-6307(03)00045-1

Engineering Failure Analysis 10 (2003) 639–643

www.elsevier.com/locate/engfailanal

* Corresponding author. Tel.: +91-657-2271709; fax: +91-657-2270527.

E-mail address: sapan@nmlindia.org (S.K. Das).

2. Results

2.1. Visual examination

Some batches of die cast Zn-Al alloy used in detonator housings were found in extensively crackedcondition after a few months of storage (Fig. 1). There were other batches of the housings that did notshow such cracks.

2.2. Chemical composition

The chemical compositions were determined by atomic absorption spectroscopy (AAS) and wet chemicalanalysis. Energy dispersive spectroscopy (EDS) attached to a scanning electron microscope (SEM) wasused for elemental analysis on grain boundaries and phases. The chemical composition of one cracked andone un-cracked housing is shown in Table 1. The chemical composition of Zamac 3 alloy that was specifiedfor the fabrication of the housings is also shown for comparison.The following points are apparent from the table.

1. The aluminium contents in both the cracked and the un-cracked housing were within the specified

range.

2. The quantity of lead in the cracked housing was much higher than the specified maximum. In the un-

cracked housing, the amount of lead was the highest that was acceptable as per the specification.

Fig. 1. Photograph of detonator housings (a) good and (b) cracked housing.

Table 1

Typical chemical composition (wt.%) of cracked and un-cracked housing

Element

Cracked housing Un-cracked housing Zamac-3 alloy

Al

4.15 4.0 3.9–4.3

Pb

0.04 0.005 0.005 max.

Cu

Not detected Not detected 0.10 max

Mg

0.014 0.014 0.03–0.08

Zn

Balance Balance Balance

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3. The amount of magnesium was lower than even the minimum in both the cracked and the intact

housings.

2.3. Microstructure and microanalysis

Microstructures of the cracked housing material are shown in Figs. 2 and 3. It consists of Zn rich b phase(black area) and Al rich a phase (white area). Cracks at the a/b interface are visible in Fig. 3. The EDSmicroanalysis on the grain boundaries showed an enrichment by lead, particularly at the cracked surfaces(Fig. 4). Microstructure of the un-cracked housing is shown in Fig. 5. It has also a and b phase but thegrain sizes are smaller than those of the cracked housing. Importantly, there is no segregation of lead orother elements to the grain boundary regions.

2.4. Fractography

Fig. 6 shows the crack surface of the housing as taken by SEM. The mode of failure is typical inter-crystalline brittle fracture. EDS analysis showed a significant amount of lead segregation on this crackregion.

Fig. 2. Microstructure of cracked housing.

Fig. 3. SEM micrograph shows crack at a/b interface.

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Fig. 4. EDS spectrum showing Pb segregation at grain boundary.

Fig. 5. Microstructure of good housing.

Fig. 6. SEM micrograph of cracked surface of housing.

642 S. Kumar Das, D. Kumar Bhattacharya / Engineering Failure Analysis 10 (2003) 639–643

3. Discussion

Aluminium is the main alloying element in zinc–aluminium alloys and has a major role in increasing thestrength of the alloy by reducing the grain size. But there is a tendency of Al to segregate to the grain bound-aries. If the amount of Al is more than 4.3 wt.% impact strength is reduced whereas it is too soft if the amount isbelow 3.7 wt.% [1]. The tendency of intergrannular corrosion would be increased if Al is oxidised in a humidatmosphere. The amount of aluminium in both the cracked and the un-cracked housing were found to be withinthe specification limits. Therefore, the problem of inter-crystalline corrosion contributed to by aluminium isruled out in the present case. It was found in the cracked housing that the amount of lead was much higher thanthe upper limit of the specification range. But the amount of lead in the un-cracked housing was within thespecification. Lead can be particularly dangerous if it exceeds the specification limit. If the atmosphere is humid,lead facilitates inter-granular corrosion of Zn–Al alloy even at room temperature. In the absence of aluminium,zinc does not corrode in this way. When aluminium is present, zinc begins to show pronounced susceptibility tointer-granular corrosion. The corrosion rate increases extensively if lead is present above the permissible limit, inwhich case the grain boundary regions (aluminium rich alpha phase surrounding the zinc rich beta phase) areenriched by lead through segregation. In this region, aluminium oxidises in the presence of lead which is morecathodic to aluminium. It is therefore necessary that the amount of lead is controlled below a critical amount(0.005 wt.%). An addition of small amount of magnesium (0.03 wt.%) to the alloy can prevent intercrystallinecorrosion by forming a stable compoundMgZn5. In the present case the amount of magnesium was lower thanthe specified amount in both the cracked and the un-cracked housing. However, since the amount of lead in thecracked housing was very high and that in the un-cracked housing very low, it is thought that the effect of thelower amounts of magnesium in either of the housings was not relevant.Besides controlling the chemical composition, chromium plating is done to reduce the problem of corrosion

further. In the present case, though Cr-plating was done, it was not uniform. Coating was non-existent inmany places in the cracked housing. The coating in the un-cracked housing was better than the cracked one.

4. Conclusions

Extensive inter-granular cracks in Zn–Al (Zamac 3) detonator housings was found during storage underatmospheric conditions where the humidity was not controlled. The corrosion was facilitated by the seg-regation of lead at the grain boundary regions. Lead being more cathodic as compared to aluminium, theoxidation rate of the latter increases in the presence of the former. This oxidation leads to grain boundarycracking. The amount of lead in the cracked housing was found to be much more than the specified max-imum. Though the necessity of control over chemical composition has been known for a long time, theproblem still persists. Though chromium plating was done as a means to prevent corrosion, it was noteffective, since it was not done properly.

Acknowledgements

The help provided by Mr. K.K. Gupta, of ANC Division, National Metallurgical Laboratory is grate-fully acknowledged.

References

1 Morgan SWK. Zinc and its alloys and compounds. Publisher: Ellis Worwood Ltd., 1985.

2 Jaykumar T, Bhattacharya DK, Baldev Raj, Rodrigues P. Pract Met 1982;19:539–41.

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