John F Simmons discusses the causes and prevention of propeller blade erosion and corrosion
A measure of degradation
The effect of minor roughness on propeller performance was demonstrated on a tug during the 1980s. With a new propeller fitted, the maximum bollard pull was measured. The propeller was then painted and, as it dried, compressed air was blown over it to give the surface a rippled finish mimicking roughness. The maximum bollard pull was then measured again. It was 15 per cent lower.
Causes
The ultimate cause of propeller performance degradation is a rough surface on the blades. The causes of rough surfaces are accretions of marine organisms, metal alloy erosion and corrosion, or any combination of these elements.
Accretions
Accretions of marine organisms occur when a propeller is left unused in water containing high levels of the calcium-encased organisms that attach to wetted surfaces.
There are four recommended methods of prevention. These include manufacturing the propeller from a copper-based metal alloy, frequent cleaning and polishing, frequent hard driving of the propeller to wash any organisms off before they become established, and coating the propeller with a product specified to make underwater surfaces too slippery for organisms to stay attached. If conditions dictate, all of these measures may have to be used.
Erosion
Erosion occurs when turbulence or cavitation is high enough to overcome the propeller metal alloy characteristics.
There are two recommended methods of prevention: minimising turbulence by keeping the propeller smooth, and minimising cavitation by using the worst-case load parameters when choosing the propeller design and accept lower top speed performance.
Corrosion
Corrosion occurs when the propeller is made of an unsuitable metal alloy for the vessel's environment or when it is subjected to electrolysis or galvanic action. More than one cause may be present at the same time.
There are three recommended methods of prevention:
Bronzes ain't bronzes
A study entitled The Effect of Composition and Microstructure on the Seawater Corrosion Resistance of Nickel Aluminum Bronze1 showed that a 1.3 per cent reduction in aluminium could have a significant deleterious effect on corrosion resistance. However, just as too little of some elements can cause corrosion problems, so can too much of other elements. Hence the need to specify metal alloys accurately.
Specifying propeller material by the use of a generic name (aluminium bronze) or a trade name that has become generic (NiBrAl, originally registered by The International Nickel Co) leaves the user with no recourse if the metal alloy is any aluminium bronze or in the case of nibral, any nickel aluminium bronze. It is best practice in the case of critical metal alloy components to be specific about the metal alloy standard.
However, specifying a metal alloy to a national standard also presents difficulties. If a metal alloy is specified to a particular national standard then it is likely that it is only manufactured in that nation. If the manufacturer only makes a batch of that metal alloy when there are sufficient orders to make it worthwhile, it may be that there is none available when required. That would force a change of standard at the time of ordering and/or require sourcing internationally.
The solution to this problem is to use the Unified Numbering System (UNS)2. In the subject literature for seawater applications, the nickel aluminium bronze UNS C95800 is the most widely recommended metal alloy for top quality propellers. Alternatives are C95810 and C95820, but they have fewer equivalent standards and those two standards allow higher percentages of trace elements. For example there is an Australian Standard, AS 1565-1996-C95810.
Alloy guarantee
Minimum: Specify any metal by its UNS number so there cannot be any misunderstanding about precisely what metal alloy is required.
Moderate: Require the component manufacturer to quote the UNS equivalent standard to which the metal alloy for the component will comply. If it fails under normal use have it tested by a NATA3 accredited laboratory to see if it complies with the quoted standard. If it does not, make a claim for compensation from the contractor.
Maximum: Specify that test pieces be made and tested, and certificates be provided in accordance with the quoted standard before the component is accepted. That will incur extra cost.
Case study
In the 1975-76 financial year, Australian Customs acquired three, 14-metre aluminium, twin screw, seagoing vessels. The propeller shafts and 'A' brackets were 'marine grade stainless steel' and the propellers were purported to be 'aluminium bronze'. Within six months of each vessel being launched the propellers were corroded and eroded to the extent that they required replacing.
The problem was discussed with the builder's naval architect and the propeller manufacturer. The hypothesis; propeller cavitation caused by the vessel having a higher operating load profile than they anticipated. As a result it was decided to replace the original 610 by 610 millimetre pitch propellers, with 610 millimetre by 559 millimetre pitch propellers from the same manufacturer. The replacement propellers suffered the same fate.
The next hypothesis; stray electrical currents caused the problem, despite there being no external hull corrosion and normal corrosion rates on the hull anodes. To test that hypothesis one of the vessels was fitted with shaft brushes and bonding on one shaft, and a shaft anode was fitted to the other. The experiment demonstrated that the brushes slightly damaged the shaft; the anode quickly corroded and eroded away and came loose on the shaft; and, the propeller corrosion and erosion continued. Further research was required.
After a literature review it was decided to determine the metal alloy composition of the failed propellers. Metallurgists at the Royal Australian Mint undertook the task. They cut a large piece out of a blade and polished and etched the edges and photographed them under a microscope at magnifications of 200:1 and 500:1. Individual zinc crystals were identified under the microscope and in the photos. They also did a chemical analysis and determined that the material was indeed an 'aluminium bronze' but that it did not meet any published standard for that metal alloy. Notably, it had zinc content in excess of the amount allowed by any of the relevant standards.
In light of the Mint report it was concluded that the mechanisms of failure were:
At that stage it was decided to obtain competitive quotes for propellers manufactured from nickel aluminium bronze to British Standard 1400 AB2; the best standard metal alloy at the time (now superseded). Neither the resulting propellers nor others ordered to BS1400 AB2 ever suffered any corrosion or erosion problems.
Case study continued
The literature review also showed that the Galvanic Series in seawater has the aluminium bronzes located between 316 stainless steel (UNS S31600) in its passive state and the same metal alloy in its active state4. Therefore, depending on environmental conditions the same stainless steel shaft can be the cathode or the anode to a nickel aluminium bronze propeller. To ensure all round protection it was decided to fit anodes on that running gear.
As zinc had proved too soft, and it was suspected that aluminium would also be too soft it was decided to make the anodes from the next most anodic material in the series, low carbon (mild) steel. As it is more anodic than both active stainless steel and nickel aluminium bronze, it would protect the shaft and the propeller. To avoid the problem of an anode on the shaft coming loose, or causing crevice corrosion if it remained tight, it was decided to fit anodes in the form of cone nuts.
From the first slipping onwards the shafts had exhibited pitting corrosion, after fitting the mild steel cone nuts that stopped. The hulls never suffered underwater corrosion but the zinc anodes on the transom did and were replaced as necessary.
Case study concluded
Over the next 20 years, customs specified propellers made of nickel aluminium bronze to BS 1400 AB2 and the fitting of mild steel cone nuts, for aluminium and for fibre reinforced plastic (FRP) vessels. Over that time propeller performance degradation only occurred when propellers were not kept clean.
During the late 1970s the Royal Australian Navy (RAN) was having similar problems with the propellers on FRP fast personnel carriers. Based on Customs' experience, the RAN applied the same solutions to one of those vessels. They were shown to be successful within three months and were then applied throughout that flotilla.
No guarantees
"But corrosion engineers, like economists, know enough to provide plausible explanations of what has happened without being equally adept at predicting future occurrences5."
Final advice
Don't jump to conclusions about the causes of propeller performance degradation.
John F Simmons
1 The study can be found here
2 See Metals and Metal Alloys in the Unified Numbering System (UNS): 11th Edition, ASTM International, West Conshohocken, 2008. The Unified Numbering System for ferrous and non-ferrous metal alloys identifies equivalent standards for any given metal alloy by giving it the same UNS Number irrespective of what organisation issues the standard. The UNS does not include specifications for metal alloys; they are contained within the standards
3 The National Association of Testing Authorities (NATA)
4 FL LaQue, Marine Corrosion Causes and Prevention, John Wiley & Sons, New York, 1975. For up-to-date confirmation click here