Stress Corrosion Technical Bulletin

Transcription

1 Stress Corrosion Technical Bulletin Table of contents Foreword Background Cause and Result Recommendations 4.1 Post Seamer Rinsing 4.2 Can Warmer / Pasteuriser 4.3 Can and End Rinsing 4.4 Can and End Drying 4.5 Secondary Packaging 4.6 Storage 4.7 Treatment of Filled Product Affected by Stress Corrosion 4.8 Transportation of Filled Product 5. Test Method for Assessing the Dryness of Can Ends 5.1 Equipment Required 5.2 Test Procedure 5.3 Test Frequency 5.4 Points to Note 5.5 Standard to be Achieved 6 Description of Figures Appendix 1 Page 1 of 13 ISSUE: October 2003

2 1. Foreword The recommendations below have been formulated to minimise the risk of stress corrosion on aluminium easy open ends used in the packaging of beverages in aluminium and steel 2-piece beverage cans. Its compilation has involved the experiences and input of can makers and their customers within Europe, and it is published as a recommend code of practice jointly by the Beverage Can Makers Europe (BCME) and the UK Metal Packaging Manufacturers Association (MPMA). 2. Background Under normal conditions of use, aluminium alloy beer and beverage Ends perform without any issues. However, under certain filling and storage conditions, external corrosion can take place in the score area of the end causing premature opening of the score panel. The cause of this failure is commonly referred to as stress corrosion, but is otherwise known as Trans-granular Stress Corrosion (TGSC). Stress corrosion typically takes place in products packed in shrink-wrap packaging during storage in warehouses, but can also occur in non shrinkwrapped packages. The risk of stress corrosion rises dramatically in hot and humid climates, typically +30deg. C and humidity +60% relative humidity (rh). Depending on conditions, described later, failure can occur in a matter of days, or take several months. Visible tab staining can be an indicator of the presence of stress corrosion, even though the corrosion mechanism may not yet have developed to the point where the score panel has opened. For a better understanding of stress corrosion, please refer to the Figures at the end of this Technical Bulletin. 3. Cause and Result The corrosion takes place in the score area of the End due to the presence of moisture and the resultant chemical reaction with the aluminium alloy. The ongoing corrosion occurs at the base of the score, weakening the residual metal. This corrosive weakening, together with the internal Can pressure and the inherent stress created within the End during manufacture, causes the premature opening of the score panel. The corrosion process is greatly accelerated by the increased presence of salts (halides {e.g. chloride}/nitrates/sulphates) which can remain on the End or be present in any residual rinse water after inadequate drying of the Can body and End. Failure of one can in a shrink-wrapped pack will normally release moisture and consequently initiate corrosion in the remaining cans in the pack. Appendix 1 Page 2 of 13 ISSUE: October 2003

3 4. Recommendations BCME / MPMA strongly recommend the following precautions to be taken to minimise the risk of stress corrosion: 4.1 Post Seamer Rinsing The Can and End should be thoroughly rinsed after seaming to remove any product or process residues. The rinse water supply should ideally be electrically interlocked to the Seamer such that the line cannot run without rinse water sprays being operational. Note: Conveyor / Track lubricant (soap) and/or product carryover to the warmer water can contribute to corrosion. Build-up of these contaminants in the water can be prevented by efficient rinsing prior to entry. 4.2 Can Warmer/Pasteuriser The use of a can warmer/pasteuriser is recommended to ensure that the product temperature at the time of secondary packaging is above the dew point. This will help prevent condensation from forming on the Cans and Ends during subsequent warehousing. The presence of salts such as chlorides and sulphates accelerate the stress corrosion process therefore management and analysis of water quality in this area is essential. 4.3 Can and End Rinsing Regardless of whether the filled Can has been through a can warmer/pasteuriser, both the Can body and End must be thoroughly rinsed prior to secondary packaging to remove any salts, track lubricant, etc. This should ideally take place as near as possible to the secondary packaging station but immediately prior to the Can and End drying station. The Cans and Ends should always be rinsed in single file. It is recommended that the rinsing takes place on the final can twist prior to the drying station via fan-type nozzles ensuring that no contaminants remain on the Can or End. The rinser sprays should ideally be electrically interlocked to the can conveyor control such that if the rinser is not operational the conveyor will not run. If it is necessary to use track lubricants, these should have low chloride/sulphate levels and be minimally applied at all times. The concentration level of track lubricant should be minimised and controlled on an ongoing basis, plus ideally interlocked with line controls to prevent excessive foaming. All evidence of track lubricant on the Can/End must be removed at the final rinse station. The rinse water ideally should have a ph within the range 6 8. Chloride levels should be < 10ppm and Sulphates and Nitrates individually < 15ppm. Where water treatment is required, water softening by reverse osmosis is the preferred option. Regular checks on water quality should be maintained at this critical application point. Appendix 1 Page 3 of 13 ISSUE: October 2003

4 4.4 Can and End Drying The objective is to achieve zero residual moisture on the Can and End after the drying operation and prior to secondary packaging. This applies most particularly to the end and, most importantly, its score area. Score Corrosion does not occur on dry ends The cans and ends should always be dried in single file. This is more effective than mass drying. The dryer should be electrically interlocked to the can conveyor control such that if the dryer is not operational the conveyor will not run. It is strongly recommended that the efficiency of dryers is checked routinely (see: Test Method for Assessing the Dryness of Can Ends). It is also recommended that the dryer is subject to an effective maintenance programme to maintain its optimum performance. BCME / MPMA consider that Can and End Drying is a critical element of the beverage filling operation. The responsibility for adequate can and end drying rests with the filler. 4.5 Secondary Packaging Existing and future secondary packaging developments must be reviewed in conjunction with any additional actions required at the filling plants to minimise the risk of stress corrosion. Perforated shrink film will help any moisture to evaporate as will vent holes at the end of each pack. The use of board packaging will help absorb retained moisture further minimising the risk of stress corrosion. The use of plastic packaging without any cardboard will increase the risk of stress corrosion. 4.6 Storage Pallets of filled product should be stored in a manner that allows good air circulation and ease of inspection. Filled product should be inspected on a regular basis and any pallets showing evidence of leakage removed immediately. As far as possible, temperature variations within the warehouse should be avoided. 4.7 Treatment of Filled Product Affected by Stress Corrosion All packs containing any End (single or multiple quantities) with blown score panels should be removed and the entire pack scrapped, along with any other affected stock. Do not attempt to salvage any wet cans as it is a recognised fact that that these cans, due to product contamination leading to secondary corrosion, have a high risk themselves of leaking at a future date. Appendix 1 Page 4 of 13 ISSUE: October 2003

5 4.8 Transport of Filled Product Pallets should be transported such that there is adequate ventilation and room for air circulation. Use of block stacking in containers is not recommended. The foregoing guidelines are intended to help minimise the risk of secondary corrosion occurring. For further help and guidance please contact your local supplier. Appendix 1 Page 5 of 13 ISSUE: October 2003

6 5. Test Method for Assessing the Dryness of Can Ends 5.1 Equipment Required i) A balance accurate and repeatable to at least 0.001g. ii) Air tight containers and swabs: either: Sterilin tubes and cotton wool buds or: Sealable polythene bags and filter papers. 5.2 Test Procedure i) Weigh swabs/filter papers in the sealed containers to an accuracy of 0.001g and record weights. ii) Take samples from the line directly after the dryers and swab immediately. iii) Carefully use the swab/filter paper to remove all residual water from each individual End, including under the tab and in the countersink and score. iv) Reseal the swab/filter paper into the container. v) Reweigh container and swab. Record the weights. vi) The difference in weights is taken as the amount of moisture remaining on the end. 5.3 Test Frequency Recommended daily, but should be continually reviewed dependent on consistency of results and process capability. 5.4 Points to Note i) The time between sampling and testing should be kept to a minimum. ii) If drying in single lane, take 4 samples from each lane. iii) If drying in bulk, take 4 samples evenly across the belt. iv) Prevention of further contamination of the items to be weighed is important to avoid spurious results. v) The swabs/filter papers should be out of their containers for the minimum time between taking weights to prevent moisture pick-up or loss. Appendix 1 Page 6 of 13 ISSUE: October 2003

7 5.5 Standard to be Achieved The objective is to achieve zero residual moisture on the Ends but the following is a minimum practical operating standard based on available modern equipment. It is recommended that this specification is used when purchasing new drying equipment. Recommended Operating Standard Target average less than 3mg per End Action Single End 9mg or greater Establish cause & rectify. Recheck to ensure standard is achieved 4 can average 7mg or greater per End Establish cause & rectify. Recheck to ensure standard is achieved Visual checks should be carried out on an ongoing basis to verify drying performance is maintained. 6. Description of Figures Fig. 1 Typical Configuration of Stay-on-Tab Easy-open-End and Score Fig. 2 Environmental Stress Crack Development in Aluminium End Score Fig. 3 Environmental Stress Cracking: Fracture Surface of Aluminium End Score. Fig. 4 Fracture of Aluminium End Score Showing Fatigue Effects. Fig. 5 Fracture of Aluminium End Score Physically Induced by Pressurising to Bursting Strength. Fig. 6 Tab Staining on Ends Affected by Stress Corrosion. Appendix 1 Page 7 of 13 ISSUE: October 2003

8 Fig.1: Typical Configuration of Stay-on-Tab Easy-Open-End and Score B B Figure 1a EO End Exterior Plan View Figure 1b Scored Region Material Thickness Score Residual Thickness Section B - B Figure 1c Score Section profile Appendix 1 Page 8 of 13 ISSUE: October 2003

9 Fig.2: Environmental Stress Crack Development in Aluminium End Score Score region shown in Figure 2 Score Residual Thickness Material Thickness Environmental stress crack showing typical branching progressing from pit beneath score exterior. Secondary sites of corrosion and crack development are present at the left-hand edge of the score. Field of view width 200µm. Appendix 1 Page 9 of 13 ISSUE: October 2003

10 Fig. 3: Environmental Stress Cracking: Fracture Surface of Aluminium End Score Top of score residual Secondary cracks Fan shaped cleavage cracks Inside edge of score residual Environmental stress cracking: Surface of score residual fracture showing fan shaped cleavage cracks. Secondary cracks typical of environmental stress cracking are also visible. Field of view width 230µm. Appendix 1 Page 10 of 13 ISSUE: October 2003

11 Fig. 4: Fracture of Aluminium End Score Showing Fatigue Effects Top of score residual Striations Inside edge of score residual Fracture surface of aluminium end score showing horizontal striations indicating fatigue effects. Field of view width 180µm. Appendix 1 Page 11 of 13 ISSUE: October 2003

12 Fig. 5: Fracture of Aluminium End Score Physically Induced by Pressurising to Bursting Strength Top of score residual Ductile cups and cones visible Inside edge of score residual Fracture surface of unused aluminium end score blown under pressure showing ductile cup/cone physical failure effects. Field of view width 190µm. Appendix 1 Page 12 of 13 ISSUE: October 2003

13 Appendix 1 Page 13 of 13 ISSUE: October 2003