(A Slim Monograph by Barrie Gillings)
This booklet is published by the Derby Phantom Register of the Rolls-Royce Owners’ Club of Australia (NSW) Inc., Box 2754 G.P.O. Sydney, NSW 2001 Australia. ABN: 42327 667 396 Whilst all care is taken, the information and advice in this booklet is the record of the experiences or views of its authors and are not necessarily endorsed by the RollsRoyce Owners’ Club of Australia (New South Wales Branch) Inc, the Club’s officials or its individual members. Advice given in this booklet is gratuitous and readers should seek an independent opinion. The Club and the authors cannot be held responsible for any information, advertisement or transaction published in, or begun through this booklet.
HOW THIS BOOKLET CAME TO BE
In recent years, the Registrars of various Registers have arranged for seminars to be held at RROCA Federal Rallies. For the 54th Federal Rally “Ecstasy in Paradise”, seminars were conducted for the Pre-war Small Horsepower Register, the Silver Wraith, Silver Dawn, Mk VI and R Type Register and the Derby Phantom Register. Brian Crump, the Registrar for the Derby Phantom Register, asked me to give a talk on aluminum cylinder head corrosion, a subject of considerable concern to me, as my Phantom II has had this problem. Because of my scientific background, I conducted a series of experiments, and did some research, which I thought might be of interest to fellow owners.
A small but enthusiastic group of Phantom owners attended my presentation, and some were sufficiently interested to ask for a digest of it. Brian Crump urged my wife Margaret and me to prepare a booklet covering the subject, and this record of my research, its related physics and chemistry and what I discovered during my cylinder head restoration is the result.
Some readers may feel that the areas I covered are surplus to their requirements, and may be tempted to skim-read them or by-pass them entirely. I urge them to persevere, because a well-informed owner is much less likely to be bamboozled by an opportunistic R-R or B car salesman, or even worse, a hungry ‘restoration specialist’.
I have written the areas covered in this booklet in a way which I hope allows understanding without the need for exam-passing skills in Physics I or Chemistry I, so there is no quiz at the end. But if, having read it, you understand that radiator water should be as pure as you can afford, that you should not add ethylene or propylene glycol (unless you live where rivers and creeks freeze) and that anything you do add is simply to suppress corrosion and foaming, this booklet will have achieved its aim.
THE BASIC SCIENCE
The Periodic Table lists the chemical elements according to the number of electrons orbiting their atoms. The first is Hydrogen (H1), which consists of a positively charged Proton, with one negatively charged orbiting Electron. (Fig 1, left). The next is Helium (He2) right, with two protons, and two orbiting electrons. Lithium (Li3) is next, with three protons and three electrons, but its third electron starts a new ‘layer’ of electrons. Beryllium (Be4) is next with two outer shell electrons. On the right are Boron, Carbon, Nitrogen, Oxygen, Fluorine and Neon, with 3, 4, 5, 6, 7 and 8 outer shell orbiting electrons. It is the matching Protons (and Neutrons) in the atoms that give them weight, and the outer ‘layer’ of electrons which dictate the element’s chemical behaviour. Element 11, Sodium, starts another new layer of eight electrons, and this layer of eight ends with element 18, Argon (A). The next layer can accommodate 18 electrons, starting with 19, Potassium (K), and ending with 36, Krypton (Kr).
The periodic table arranges the elements in recognizable groups, shown here separated by dark lines with the light metals on the left, the inert gases (which have complete electron layers) on the right, high melting metals in the middle, (further separated into brittle, ductile, noble and low-melting) and non-metals on the right. The ‘complete’ layers, or ‘shells’ of orbiting electrons are maximums of two, then eight, eight, eighteen, eighteen and eighteen The chemical and electrical behaviour of elements depends entirely on the behaviour of the outer layer of orbiting electrons, and their weights the number of Protons and Neutrons in their nucleii. Only the outer electrons take part in chemical reactions.
The atoms of metals are solid at room temperature (except Mercury (Hg)) and arrange themselves, through mutual attraction, in the crystalline patterns of face-centered cubic, body-centered cubic or close-packed hexagonal. Melting destroys these patterns, and when cooling, the atoms re-assemble in their specific crystalline patterns. The individual crystals grow larger as they cool and accumulate more atoms, until eventually they come up against a neighboring crystal and growth ceases. These interfaces, which are three-dimensional, are called grain boundaries.
If an appropriate acid is applied to a polished metal surface, the grain boundaries are preferentially etched, and display the grain structure. This technique reveals much about the nature of the metal or alloy, and helps to explain its behaviour.
Few metals are used in the pure state because mixtures of them, called alloys, often have superior qualities. For example, bronze, an alloy of copper and tin, is harder and tougher than the parent metals. Metals in alloys almost always have different melting points, so what happens when a molten mixture of two metals is cooled? The graph in Figure 4, upper left, plots temperature (vertical scale) against various mixtures of nickel and copper, from 100% nickel on the left to 100% copper on the right. Above the upper graph line, the alloy is in the molten state, and the below the lower graph line is in the solid state. In the space between the two graph lines the alloy is in a ‘mushy’ or ‘pasty’ state, neither solid nor liquid. This type of alloy is termed a ‘solid solution’
A different form of alloy is the ‘eutectic’. The graph in Figure 4, upper right, plots temperature against various mixtures of 100% lead, on the left, and 100% tin on the right. The graph demonstrates that at one point in the mixture range, the melting point is lower than the melting points of the two components, and such alloys are called ‘eutectics’. This alloy of 62% tin and 38% lead is ‘electricians’ solder’, and solidifies quickly and at a lower temperature than either lead or tin.
Grain size shows great variation in different metals and alloys. Figure 5 shows grain size changes after brass is cold-rolled and annealed Figure 5 .
This demonstrates how the grain size and shape of a metal or alloy can be changed by cold-working and later annealing. Cold-working usually increases hardness and strength but reduces ductility while annealing causes grains to grow and restores ductility, but reduces strength.
Figure 6 Diagram of an electrolytic cell.
Corrosion occurs in two forms: chemical and galvanic, alone or together. Chemical corrosion occurs when a liquid or gas reacts with a metal surface. Galvanic corrosion occurs when two dissimilar metals are separated by an electrolyte and there is present an electrical current pathway. The freedom with which the outer electrons of the metal or alloy can be released or interact governs the nature of the corrosion. For the Phantom II cylinder head, the galvanic corrosion is the more destructive. But chemical corrosion does occur, and is most commonly the result of oxygen reacting chemically with the metals of interest. Aluminium, exposed to air, becomes coated with a very adherent and protective coating of aluminium oxide, which can protect against further corrosion. Manufacturers can deliberately induce this oxide formation on aluminium, usually incorporating pigments, in the process of ‘anodizing’. Magnesium, zinc, chromium, nickel, titanium, tin and cadmium all form oxides which are, more or less, protective against further corrosion in air, and are self-limiting. Other chemicals in the air such as sulphates, carbonates, sulphides etc. can form somewhat protective coatings, such as the verdigris on copper and bronze. For the destructive galvanic corrosion to occur, you need an anode, a cathode, an electrolyte and a current pathway, as shown in the diagram. Eliminate any one, and you stop this corrosion. Painting the anode or cathode provides insulation: no current pathway. Cut the contact between anode and cathode, again insulation: no corrosion. Remove the electrolyte (dry the surfaces): no corrosion. In some cases, a deposit of corrosion products on the ‘electrode’ isolates it from the electrolyte, acting like a coat of paint, and this initial corrosion then progressively prevents further corrosion. Unfortunately, this protection rarely protects the PII cylinder head.
The likelihood of galvanic corrosion can be assessed from the ‘electrode potentials’ of the metals/alloys involved. These potentials are measured as voltages +/- in comparison with a standard electrode (hydrogen gas bubbled over a platinum electrode), and recorded as the ‘electromotive series’. This table records the potentials for a range of elements, (but omits an important one, Magnesium, (-2.4)). From this table, you might assume that if you have Copper (+0.47 volts) and Zinc (-0.76 volts) in a conducting solution, you have a battery developing 1.23 volts. This is an over-simplification, because surface chemistry is also involved. But in broad terms, if the two metals are joined, the Copper will acquire a surface layer, depending on the electrolyte, and the Zinc will corrode. Likewise, coat Iron (-0.44 volts) with Zinc (-0.76 volts) and immerse them in a conducting solution and the Zinc will corrode preferentially and protect the Iron. This is the rationale of galvanised Iron, with the Zinc protecting the Iron ‘galvanically’. Magnesium blocks bolted to outboard motors protect the motors by dissolving preferentially in seawater as a ‘sacrificial anode’, so you could say that the Phantom II cylinder head aluminium corrodes to protect the iron, nickel, copper, chromium, lead, nickel and cadmium components.
An important factor in the corrosion of PII cylinder heads is the nature of the coolant. Any soluble salts (which dissociate into anions (+ve) and cations (-ve)) in the water will make it conductive, thereby promoting electrolytic corrosion. So will any strong acid or alkali, but these may also promote chemical corrosion. So use distilled or deionised water, or clean rainwater collected from a tile, plastic or painted (not galvanised iron) roof, in your radiator. Do NOT use tap water, which has dissolved salts, even though it might taste just fine. It is important that radiator water not be strongly acid or alkaline. You should have a general idea of the acidity or alkalinity (pH) of your water. pH stands for “the power (p) of the Hydrogen ion (H+ ) concentration, and is not too hard to understand. Pure water is two Hydrogen atoms joined to one Oxygen atom, as H2 O. The three atoms are firmly bound as a molecule, but in pure water, one in ten million separates into one Hydrogen ion H+ and one Hydroxyl (Hydrogen/Oxygen) ion OH-. This state is recorded, by convention, as the concentration of the hydrogen ion, one part in ten million, 1 in 10-7 .which can be expressed as a negative logarithm of the concentration, ie, 10- 7 , and is called pH7. If you increase the H+ concentration tenfold, by adding, say, a very little hydrochloric acid (H+Cl-) to the water, the pH becomes 6; by 100, pH 5, a thousand; pH 4 and so on until pH 0, when all the solution is pure H+ and Cl-. Likewise, if you increase the OH- concentration by ten, by adding very little caustic soda (Na+OH-) say, to the water, the H+ concentration decreases to 1 in 100 million, or pH 8 and so on. The pH values for many common solutions are shown in the table, and you will find that most radiator water will be like swimming pool water, say 7.5 to 8.5, especially if you have added commercial coolant additives like Penrite or Water-Wetter. What you do not need in the radiator water is ethylene or propylene glycol. These are excellent wetting agents, and will find even very small leaks in your system. So unless you live above the snow line, in winter, the glycols are not needed. In addition, they are considerably less efficient than pure water in transferring heat from the engine to the radiator.
CORROSION IN CYLINDER HEADS
Rolls-Royce stressed the need for smooth finishes on its cast or machined metal components. There was a reason for this. If there is a crack (or scratch or pit) on a surface, and the surface is wet with a fluid containing ions, a corrosion cell is formed. The bottom of the crack acts as the anode, and the edges of the crack act as the cathode. An ionic current runs through the electrolyte, and an electric current through the metal. This causes the situation shown in Figure 6. Thus the smoothness of surfaces on, say, a connecting rod has a definite benefit. There is another, related benefit. Analysis of materials under stress shows a very heavy concentration of these stresses on large scale sharp internal corners and bends. Small scratches, cracks and pits can cause an even greater concentration of stress and possible fracture. A polished surface eliminates this effect.
Here are (l-r) a Phantom II rocker cover, cylinder head, the two blocks, the twelve cylinder block cover-plates, then the front head cover-plate and the fan assembly. These have all been chemically cleaned and bead-blasted. When undertaking a cylinder head restoration, the head chemical cleaning and bead blasting gr