One of the established misunderstandings associated with the art of decorative chromium plating is the tagging of everything emerging from the hexavalent chromium electroplating tank with white or gray appearance in the high-current density (CD) areas as “burned.” In addition to the confusion it causes, considerable time and efforts are dissipated looking for the solution to the problem in those areas, which are not necessarily related to a true burn. In actuality, so-called burns from a chromium bath can be subdivided into two categories:
- A True Burn
- A False Burn (sometimes colloquially termed “chicken tracks” and “whitewash”)
A true chromium burn could be defined as one occurring at any high CD area. It is characterized by heavy gray-colored chromium deposit, which fades out, becoming a lighter and lighter gray until it blends into bright chromium deposit as presented in Figures 1-3. Figure 3 makes a direct comparison of true and false burns occurring on the same areas of two identical chromium plated parts. A true burn usually is solved by appropriate corrections in the current density, time, temperature, racking and ratio relationship within the chromium bath itself, or with elimination of rectifier problems. At this point, since it is more straightforward, we will leave the subject of a true burn for another place and another time.
A false chromium burn is characterized by jagged, white areas of chromium, irregular in pattern, with sharply defined edges, occurring on the higher CD areas during chromium plating. It can also contain bright patches of chromium within these cloudy jagged patterns as presented in Figures 4-6.
These are the basic forms of true and false burn. At times differentiation between the two is difficult, since both may occur simultaneously. Experience is now necessary and mandatory.
Since all false burns are similar in appearance, there is a strong tendency to believe that all originate from the same source. Actually, there are many different conditions that can cause false burns. This is one of the reasons that there is so much mystery and confusion about burns.
The plater is often perplexed and frustrated when the false burn is the cascading result of two or more phenomena, each contributing a small share to the total problem. As each one is corrected, there may be no noticeable improvement unless it is the major problem, or perhaps, the last remaining portion of the combined problem to be solved.
Since visually one can seldom separate and identify the cause of a false burn, the position on the rack can sometimes be an important clue as to the origin of the problem, as in the case of ubiquitous bipolar effects1, 4.
Four Sources for False Burns
False burns usually originate from four major sources:
- Electrical problems.
- The nickel solution.
- Difficulties with tanks or processing between nickel and chromium.
- The chromium solution.
The complicating factors interwoven between any of the above could be of chemical, mechanical or electrical nature, resulting in a plurality of areas to be investigated. Further complicating the problem is the fact that any change in the activity of the nickel underplated surface intensifies the effects of any electrical, mechanical or chemical condition, to cause a false burn.
It can be expressed in non-electrochemical jargon like two wildcats being thrown together. Action on the part of one causes an immediate reaction on the part of the other and separating these problems, “is like sorting wildcats.”
Primarily, one cannot understand and troubleshoot decorative bright chromium electroplating without being knowledgeable about conditions, which relate to the underlying bright nickel deposit and the activity of the nickel surface. However, it is possible to understand bright nickel without concerning one’s self with the chromium topcoat. The reasons are simple and relate to those conditions that cause an invisible film of passivation or oxidation on the nickel surface. In the sequence of operations, whereby chromium is the last significant process in the line, we could metaphorically articulate that the chromium plating process is in a similar position to “the only person seen to leave the scene of the crime.”
As passivating films are normally invisible by simply looking at the nickel-plated surface prior to chromium plating, they usually give no advance indication of possible troubles. However, the work emerges from the chromium tank with the “burn that burns.” Genuinely, it could be that the chromium bath is at fault. However, to arbitrarily assume the problem is in the chromium plating operation because the reject was noted after this operation is an assumption that a suspecting or unsuspecting electroplater may have regret. There is no harm in making that assumption, only in acting on it when not having the right facts and wisdom to back it up.
A Magic Addition?
It is often tempting for platers to correct plating problems by the addition of excess of plating salts and additives to chromium and nickel baths. Unfortunately, this is a wild guess and a shot in the dark and the plater generally gets the ricochet. The addition more often than not leads to chemical imbalance and creation of another problem. Alas, now there are at least two problems.
We hasten to add that there is nothing wrong with adding the correct amount of chemicals to reestablish normal operating parameters. In fact, this may help to correct the problem. In this case, respects on being an intelligent plater.
Separating a chromium problem from a nickel problem can be complicated. Even the most experienced technician or plater may be frustrated in his attempt to do this. As you may have guessed by now, our frustration and anger resulted in the title of this article.
Considering some of the reasons why chromium plating is difficult to understand, one can point to some of the idiosyncrasies of chromium plating itself. For example, chromium plating processes have cathode efficiency in the region of 12-22 %. They have poor covering power in relation to other common plating processes. The main ingredient is chromic acid, which is a strong oxidizing material. Chromium plating uses insoluble lead alloy anodes. They require proper filming for freedom from scale for good current distribution and for the continuous reoxidation of trivalent to hexavalent chromium. The quantity of primary catalyst (sulfate) that is necessary to promote good chromium plating is quite small: 1/2 or 1% of chromic acid. Therefore, small errors in analysis can affect the CrO3/SO4 ratio and quality and quantity (speed) of the chromium deposit.
Additionally, chromium and nickel baths, even when operated in good chemical balance, are sensitive to any condition that passivates or oxidizes the surface of the bright nickel deposit. Those conditions can be numerous, e.g. long transfer times between nickel and chromium tanks, stray currents, bipolarity, poor rinsing and contaminated rinses.
Carrying this whole ugly mess just a few steps further, we might note that chromium solution is inherently messy. It stains and eats holes in clothes, looks terrible on the hands at dinner parties, weddings and confirmations. In addition, in fact, it generally makes itself so ugly that most people prefer to talk about it but keep a moderate distance from the tank.
Nevertheless, a chromium plating bath in respect to the number of ingredients involved is about as simple a formulation as the alkaline stannate bath, since it contains no organic brightening agents, buffers, complexants, etc. Thus chromium solutions should require a minimum of thought and control to keep them operating satisfactorily over a reasonable period of time.
The complications in a bright nickel-chromium system are actually in the lack of proper understanding of the interlocking relationship and sensitivity of chromium to nickel and vice versa.
Leave Well Enough Alone
This attitude applies not only to chromium baths in the majority of cases, but to other baths as well. If a recent analysis is indicating the chromium bath to be in proper chemical balance and the Hull Cell demonstrates normal coverage and deposit properties, then it is best not to disturb the bath. The problem should not be complicated by an unnecessary addition of chemicals or by changing standard operating conditions. Instead, diligent watch for the problem in the fundamentally indicated areas is required. Otherwise, the plater may become a protégé of the Headless Horseman, who allegedly tried to ride off in all directions at once.
If you, as readers, are discouraged at this point, please don’t be. This is only necessary background information to help you more thoroughly understand the rationale and explanations that follows and to make you pause and contemplate before initiating unnecessary changes in this or any plating systems. In next month’s continuation of this article, we’ll begin our in-depth analysis of the four sources for false burns.